=Paper= {{Paper |id=Vol-3859/paper5 |storemode=property |title= Do We Trust What They Say or What They Do? A Multimodal User Embedding Provides Personalized Explanations |pdfUrl=https://ceur-ws.org/Vol-3859/paper5.pdf |volume=Vol-3859 |authors=Zhicheng Ren,Zhiping Xiao,Yizhou Sun |dblpUrl=https://dblp.org/rec/conf/mmsr/Ren0S24 }} == Do We Trust What They Say or What They Do? A Multimodal User Embedding Provides Personalized Explanations == https://ceur-ws.org/Vol-3859/paper5.pdf
                                Do We Trust What They Say, or What They Do? A
                                Multimodal User Embedding Provides Personalized
                                Explanations
                                Zhicheng Ren1,⇤ , Zhiping Xiao2,⇤ and Yizhou Sun3
                                1
                                  Aurora Innovation, 280 N Bernardo Ave, Mountain View, CA 94043
                                2
                                  University of Washington, 1410 NE Campus Pkwy, Seattle, WA 98195
                                3
                                  University of California, Los Angeles, Los Angeles, CA 90095


                                           Abstract
                                           With the rapid development of social media, the importance of analyzing social network user data has also been
                                           put on the agenda. User representation learning in social media is a critical area of research, based on which we
                                           can conduct personalized content delivery, or detect malicious actors. Being more complicated than many other
                                           types of data, social network user data has an inherent multimodal nature. Various multimodal approaches have
                                           been proposed to harness both the text (i.e. post content) and the relation (i.e. inter-user interaction) information
                                           to learn user embeddings of higher quality. The advent of Graph Neural Network models enables more end-to-end
                                           integration of user text embeddings and user interaction graphs in social networks. However, most of those
                                           approaches do not adequately elucidate which aspects of the data – text or graph structure information – are
                                           more helpful for predicting each specific user under a particular task, putting some burden on personalized
                                           downstream analysis and untrustworthy information filtering. We propose a simple yet effective framework called
                                           Contribution-Aware Multimodal User Embedding (CAMUE) for social networks. We have demonstrated with
                                           empirical evidence, that our approach can provide personalized explainable predictions, automatically mitigating
                                           the impact of unreliable information. We also conducted case studies to show how reasonable our results are. We
                                           observe that for most users, graph structure information is more trustworthy than text information, but there
                                           are some reasonable cases where text helps more. Our work paves the way for more explainable, reliable, and
                                           effective social media user embedding, which allows for better-personalized content delivery.

                                            Keywords
                                            Multi-modal representation learning, Social network analysis, User embeddings




                                1. Introduction
                                The advancement of social networks has placed the analysis and study of social network data at the
                                forefront of priorities. User-representation learning is a powerful tool to solve many critical problems in
                                social media studies. Reasonable user representations in vector space could help build a recommendation
                                system [1, 2], conduct social analysis [3, 4, 5], detect bot accounts [6, 7, 8], and so on. To obtain user-
                                embeddings of higher quality, many multimodal methods are proposed to fully utilize all types of
                                available information from the social networks, including interactive graphs, user profiles, images, and
                                texts from their posts [9, 10, 11, 12]. Compared with models using single modality data, multimodal
                                methods utilize more information from social media platforms. Hence they usually achieve better
                                results in downstream tasks.
                                    Among all modalities in social networks, user-interactive graphs (i.e., what they do) and text content
                                (i.e., what they say) are the two most frequently used options, due to their good availability across
                                different datasets and large amounts of observations. The graph-neural-network (GNN) models [13,
                                14, 15] makes it more convenient to fuse both the text information and graph-structure information of
                                social-network users, where text-embeddings from language-models such as GloVe [16] or BERT [17] are
                                usually directly incorporated into GNNs as node attributes. Although those approaches have achieved

                                CIKM MMSR’24: 1st Workshop on Multimodal Search and Recommendations at 33rd ACM International Conference on Information
                                and Knowledge Management, October 25, 2024, Boise, Idaho, USA
                                ⇤
                                  All work done at University of California, Los Angeles
                                � franklinnwren@g.ucla.edu (Z. Ren); patxiao@uw.edu (Z. Xiao); yzsun@cs.ucla.edu (Y. Sun)
                                           © 2024 Copyright for this paper by its authors. Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0).




CEUR
                  ceur-ws.org
Workshop      ISSN 1613-0073
Proceedings
                                               From the
                                                graph
                                             information,
                                             Elon Musk is
                                             most likely a                          follow
                                             Republican!                                      Kevin
                                                                                             McCarthy
                                                                  retweet
                                                                             Elon
                                                                             Musk



                                                             Ivanka
                                                                               mention
                                                             Trump


                                                                                              Bernie
                                                                                             Sanders




                                                             From the text
                                                             information,
                              Elon’s tweet                    Elon Musk
                              keywords:                       could be a
                              - Silicon valley                Democrat!
                              - Hollywood
                              - Tech
                              - Clean energy
                              - …...




Figure 1: When predicting which political party Elon Musk voted in 2020, graph structure-based methods and
text-based methods might reach opposite conclusions. Top: A small subset of Elon’s activity with other Twitter
users. Bottom: Some keywords extracted from Elon’s tweets. All of which are extracted before the year 2020.


great performance in a bunch of downstream tasks [18], the text information and graph-structure
information are fully entangled with each other, which makes it hard to illustrate the two modalities’
respective contributions to learning each user’s representation.
   Researchers have already found that different groups of users can behave very differently on social
media [19]. If such differences are not correctly captured, it might cause significant bias in the user
attribute prediction (e.g., political stance prediction) [20]. Hence, when learning multi-modal user
representation for different users, it is not only important to ask what the prediction results are, but
also important to ask why we are making such predictions (e.g. Are those predictions due to the same
reason?). Only in that way, we could provide more insights into the user modelings, and potentially
enable unbiased and personalized downstream analysis for different user groups.
   On the other hand, under a multi-modality setting, if one aspect of a user’s data is not trustworthy and
misleading, it might still be fused into the model and make the performance lower than single-modality
models [21]. Consider the case when we want to make a political ideology prediction for Elon Musk
based on his Twitter content before the 2020 U.S. presidential election (Figure 1), when he has not
revealed his clear Republican political stance yet. If we trust the follower-followee graph structure
information, we can see that he is likely to be a Republican, since he follows more Republicans than
Democrats, and has more frequent interactions with the verified Republicans accounts. However, in
his tweet content, his word choice also shows some Democratic traits. Due to the existence of such
conflicting information, being able to automatically identify which modality is more trustworthy for
each individual becomes essential in building an accurate social media user embedding for different
groups of users.
   To address the above two shortcomings of text-graph fusion in social networks, we propose a simple
yet effective framework called Contribution-Aware Multimodal User Embedding (CAMUE), which can
identify and remove misleading modality from specific social network users during text-graph fusion,
in an explainable way. CAMUE uses a learnable attention module to decide whether we should trust
the text information or the graph structure information when predicting individual user attributes,
such as political stance. Then, the framework outputs a clear contribution map for each modality on
each user, allowing personalized explanations for downstream analysis and recommendations. For
ambiguous users whose text and graph structure information disagree, our framework could successfully
mitigate unreliable information among different modalities by automatically adjusting the weight of
that information accordingly.
   We conduct experiments on the TIMME dataset [21] used for a Twitter political ideology prediction
task. We observed that our contribution map can give us some interesting new insights. A quantitative
analysis of different Twitter user sub-groups shows that link information (i.e., interaction graph)
contributes more than text information for most users. This provides insights that political advertising
agencies should gather more interaction graph information of Twitter users in the future when creating
personalized advertisement content, instead of relying too much on their text data. We also observe
that when the graph and text backbone are set to R-GCN and GloVe respectively, our approach ignores
the unreliable GloVe embedding and achieves better prediction results. When the text modality is
switched to a more accurate BERT embedding, our framework can assign graph/text weights for different
users accordingly and achieve comparable performance to existing R-GCN-based fusion methods. We
pick 9 celebrities among the 50 most-followed Twitter accounts 1 , such as Elon Musk. A detailed
qualitative analysis of their specific Twitter behaviors shows that our contribution map models their
online behaviors well. Finally, we run experiments on the TwiBot-20-Sub dataset [22] used for a Twitter
human/bot classification task, showing that our framework could be generalized to other user attribute
prediction tasks. By creating social media user embeddings that are more explainable, reliable, and
effective, our framework enables improved customized content delivery.


2. Preliminaries and Related Work
2.1. Multimodal Social Network User Embedding
Social network user embedding is a popular research field that aims to build accurate user repre-
sentations. A desirable user embedding model should accurately map sparse user-related features
in high-dimensional spaces to dense representations in low-dimensional spaces. Multimodal social
network user embedding models utilize user different types of user data to boost their performance.
Commonly-seen modality combinations include graph-structure (i.e. link) data and text data [23],
graph-structure data and tabular data [24, 25, 9], and graph-structure data, text data and image data
altogether [26] [27], etc.
   Among those multi-modality methods, the fusion of graph-structure data and text data has always
been one of the mainstream approaches for user embedding. At an earlier stage, without much help
from the GNN models, most works trained the network-embedding and text-embedding separately and
fused them using a joint loss [28, 29, 30, 31]. With the help of the GNN models, a new type of fusion
method gained popularity, where the users’ text-embeddings are directly incorporated into GNNs as
node attributes [23, 32, 33].
   Despite their good performance, all existing models do not explain how much the graph structure
and the text information of specific users contribute to the final prediction results, making it difficult to
give customized modality weight for downstream analysis or recommendations. Also, if one modality

1
    https://socialblade.com/twitter/top/100
is poorly learned, it can be counter-effective to the user embedding quality, making it even worse than
their single-modality counterparts [21]. How to address this problem in a universally-learned way
instead of heuristic-based information filtering, has largely gone under-explored. Hence, we propose
a framework that not only utilizes both text and graph-structure information, but also reveals their
relative importance along with the prediction result.

2.2. Graph Neural Network
Graph Neural Network (GNN) is a collection of deep learning models that learn node embedding through
iterative aggregation of information from neighboring nodes, using a convolutional operator. Most GNN
architectures include a graph convolution layer in a form that can be characterized as message-passing
and aggregation. A general formula for such convolution layers is:

                                      H(l) = (ÃH(l 1) W(l) ) ,                                     (1)
where H represents the hidden node representation of all nodes at layer l, operator is a non-linear
        (l)

activation function, and the graph-convolutional filter à is a matrix that usually takes the form of a
transformed (e.g., normalized) adjacency matrix A, and the layer-l’s weight W(l) is learnable.
   In the past few years, GNN models have reached SOTA performances in various graph-related
tasks. They are widely regarded as promising techniques to generate node embedding for users in
social-network graphs. [13, 34, 14, 15].

2.3. Neural Network-based Language Models
The field of natural language processing has undergone a significant transformation with the advent
of neural-network-based language models. Word2Vec [35] introduced two architectures: Continuous
Bag-of-Words (CBOW) and Skip-Gram. CBOW predicts a target word given its context, while Skip-
Gram predicts context words given a target word. GloVe [16] model went beyond by incorporating
global corpus statistics into the learning process. ELMo [36] was another significant step forward, as
it introduced context-dependent word representations, making it possible for the same word to have
different embeddings if the context is different. BERT [17] is a highly influential model that is built
on the transformer architecture [37], pre-trained on large text corpora using, for example, masked
language modeling and next-sentence prediction tasks. Recently, large language models like GPT-3 [38],
InstructGPT [39], and ChatGPT have achieved significant breakthroughs in natural-language-generation
tasks. All those models are frequently used to generate text embedding for social network users.
    Our framework does not rely on any specific language model, and we do not have to use LLMs.
Instead, we use language-models as a replaceable component, making it possible for both simpler ones
like GloVe and more complicated ones like BERT to fit in. We will explore some different options in the
experimental section.

2.4. Multimodal Explanation Methods
In the past, several methods have been proposed to improve the interpretability and explainability
of multimodal fusions [40]. Commonly used strategies include attention-based methods [41, 42, 43],
counterfactual-based methods [44, 45], scene graph-based methods [46] and knowledge graph-based
methods [47]. Unfortunately, most of them focus on the fusion of image modality and text modality,
primarily the VQA task, while to the best of our knowledge, no work focuses on improving the
explainability between the network structure data and text data in social-network user embedding.


3. Problem Definition
Our general goal is to propose a social network user embedding fusion framework that could answer: 1.
which modality (i.e. text or graph structure, saying or doing) contributes more to our user attribute
prediction, hence allowing more customized downstream user behavior analysis and 2. which modality
should be given more trust for each user, and automatically filter out the untrustworthy information
when necessary, in order to achieve higher-quality multi-modal user-embedding.

3.1. Problem Formulation
A general framework of our problem could be formulated as follows: given a social media interaction
graph G = (V, E) with node set V representing users and edge set E representing links between users.
Let X = [x1 , x2 , x3 , · · · , xn ] be the text content of n = |V| users, Y = [y1 , y2 , y3 , · · · , yn ] be the labels
of those users, A = [A1 , A2 , · · · , Am ] be the adjacency matrices of G, m be the number of link types
and Ai 2 Rn⇥n , our training objective is:

                                              min E [L (f (G, X) , Y)]                                               (2)
   Here, L is the loss of our specific downstream task, and f is some function that combines the graph
structure information and text information, producing a joint user embedding.

3.2. Preliminary Experiment
To investigate the effectiveness of the existing GNN-based multimodal fusion methods in filtering
the unreliable modality when the graph structure and text contradict, we run experiments using a
common fusion method that feeds the fine-tuned BERT features into the R-GCN backbone, similar to
the approaches in [23] and [10]. We observe that this conventional fusion method fails to filter the
unreliable information for some of those ambiguous users. Table 1 shows two politicians whose Twitter
data contains misleading information, either in the graph structure or text data. While the single-
modality backbones which are trained without misleading information give the correct predictions, the
multi-modality fusion method is confused by the misleading information, hence it is not able to make
correct predictions.
   These insights revealed the importance of having a more flexible and explainable framework for
learning multimodal user embedding.


4. Methodology
We propose a framework of Contribution-Aware Multimodal User Embedding (CAMUE), a fusion
method for text data and graph structure data when learning user embedding in social networks. The
key ingredient of this framework is an attention gate-based selection module which is learned together
with the link and text data, and decide which information we want to trust more for each particular
user.
   Our framework has three main parts: a text encoder, a graph encoder, and an attention-gate learner.
The text content of each user passes through the text encoder and generates a text embedding for that
user. The embedding is then passed through a three-layer MLP for fine-tuning. The adjacency matrix of
the users passes through the graph encoder and generates a node embedding for that user. At the same
time, both the text embedding and the graph adjacency matrix pass through our attention gate learner.
The output of this module is two attention weights, ↵ and , which control the proportion of our graph
structure information and text information. Without loss of generality, if we make R-GCN our graph
encoder and BERT our text encoder, our model will be trained in the following way (Equation 3-6, also
illustrated in Figure 2):

                        H(1) = (concat(A1 + A2 + · · · + Am , BERTemb (X) W(1) )                                     (3)


                                               H(2) = (H(1) W(2) )                                                   (4)
Table 1
Left: A snippet for former U.S. Rep. Ryan Costello, his tweet text content is informative for the political Party
prediction, but his Twitter interaction graph data could be misleading.
Right: A snippet for U.S. Senator Sheldon Whitehouse, his Twitter interaction graph data is informative for the
political Party prediction, but his tweet text content could be misleading.
    Name                Ryan Costello                        Name                Sheldon Whitehouse
    Ground Truth        Republican                           Ground Truth        Democrat
    Party                                                    Party
    Sample Graph        Liked Ben Rhodes (Demo-              Sample Graph        Liked Senate Democrats
    Data                crat) 20 times.                      Data                Official Account 26 times.
                        Liked Donald Trump 0                                     Not following Donald
                        time.                                                    Trump.
                        Following Mike Quigley                                   Following Barack Obama.
                        (Democrat).                          Sample      Text    My Republican partner on
    Sample      Text    Despite Trump, Iran’s elec-          Data                the CARA bill, @SenRob-
    Data                tions & chaotic ME, some                                 Portman, writes a powerful
                        Democrats want to race                                   editorial on the success of
                        ahead with ill-conceived                                 CARA and CURES (which
                        Iran sanctions                                           provided a needed boost of
                        RT @SaeedKD: Iran’s peo-                                 funding to match CARA).
                        ple care about elections.                                Good move by Trump Ad-
                        The so-called democratic                                 ministration. Cong. @Jim-
                        fringe doesn’t - by me                                   Langevin &
    Graph-              Democrat (Wrong)                     Graph-              Democrat (Right)
    backbone                                                 backbone
    Prediction                                               Prediction
    Text-backbone       Republican (Right)                   Text-backbone       Republican (Wrong)
    Prediction                                               Prediction
    Fused Model         Democrat (Wrong)                     Fused Model         Republican (Wrong)
    Prediction                                               Prediction



                                             [e↵ , e ] = H(2) W(3)                                             (5)

                                    ↵ = softmax(e↵ ),      = softmax(e )                                       (6)
   Where H and W are hidden layers and weights of our attention gate learner, X = [x1 , x2 , x3 , · · · , xn ]
is the text content, BERTemb is the BERT encoding module, A = [A1 , A2 , · · · , Am ] is the adjacency
matrices of G and m is the number of link types.
   Then, our overall training objective becomes:


                                    min E[L((↵ + ) R-GCNemb(G)
                                             + ( + ) BERTemb(X), Y)]

   Here, acts as a regularizer to ensure our model is not overly dependent on a single modality.
   Our methods offer two levels of separation. First, we separate the text encoder and the graph encoder
to allow better disentanglement on which data contributes more to our final prediction results. Second,
we separate the learning of the downstream tasks and the learning of which data modality (i.e. text or
graph structure) we can rely on more. This makes our framework adaptable to different downstream
social media user prediction tasks. The learned trustworthiness of different modalities allows for auto-
adjustment of the weight between graph structure and text modalities, hence filtering any unreliable
information once they are discovered.
   Figure 2 shows the overall architecture of our framework, note that the graph structure encoder and
text encoder could be replaced by any other models that serve the same purposes.
                                       Transformer Encoder     Text Embedding



                                                 ……
                    Text Content




                                                                    …
                                                                                        GNN Model
                     Tweet Data                                                             with        loss
                                                                                        Relation Info




                   Graph Structure


                                     Transformer Encoder     Text Embedding



                                              ……                                Multi-Layer
                  Text Content
                                                                                Perceptron




                                                                  …
                                                                                                          loss
                   Tweet Data                                Attention Gate
                                                                Learner


                                                                                 GNN Model
                 Graph Structure                                                     with
                                                                                 Relation Info




Figure 2: The architectures of our framework. Top: Simple Fusion method for GNN (baseline), bottom: CAMUE


  We give a short complexity analysis of our architecture for the case of R-GCN + BERT: Since we are
using sparse adjacency matrix for R-GCN, the graph encoder part has a complexity of O(Lgraph EFgraph +
           2
Lgraph N Fgraph ) (according to [48]), where L is the number of layers, E is the number of edges, N is
the number of nodes, and F is the feature dimension. Since we fixed the maximum text length to
be a constant for the text encoder, it has a complexity of O(Ftext2 ) (based on [49]). Since F
                                                                                               text and
Fgraph are about comparable size, our fusion module has the complexity of O(Fgraph 2        2 ), so the
                                                                                       + Ftext
overall complexity is O(Lgraph EFgraph + Lgraph N Fgraph
                                                      2       2 ), hence we are not adding extra time
                                                         + Ftext
complexity.


5. Experiments
5.1. Tasks and Datasets
We run experiments on two Twitter user prediction tasks: 1. Predicting the political ideology of Twitter
users (Democrat vs Republican) and 2. Predicting whether a Twitter user account is a human or a bot.

5.1.1. TIMME
TIMME [21] introduced a multi-modality Twitter user dataset as a benchmark of political ideology
prediction task for Twitter users. TIMME contains 21, 015 Twitter users and 6, 496, 112 Twitter inter-
action links. Those links include follows, retweets, replies, mentions, and likes. Together they form a
large heterogeneous social network graph. TIMME also contains 6, 996, 310 raw Twitter content from
those users. Hence, it will be a good dataset to study different fusion methods of text features and graph
structure features. In TIMME, there are 586 labeled politicians and 2, 976 randomly sampled users with
a known political affiliation. Some of them are ambiguous users we investigated before. Labeled nodes
belong to either Democrats or Republicans. Note that the dataset cut-off time is 2020, so the political
polarities of many public figures (e.g. Elon Musk) have not been reviewed at that time.
5.1.2. TwiBot-20-Sub
TwiBot-20 [22] is an extensive benchmark for Twitter bot detection, comprising 229, 573 Twitter
accounts, of which 11, 826 are labeled as human users or bots. The dataset also contains 33, 716, 171
Twitter interaction links and 33, 488, 192 raw Twitter content. The links in TwiBot-20 include follows,
retweets, and mentions. To further examine the generalizability of our method, we run experiments for
Twitter bot account detection on the TwiBot-20 dataset. To reduce the computation cost of generating
node features and text features, we randomly subsample 3, 000 labeled users and 27, 000 unlabeled
users from the TwiBot-20 dataset, and form a new dataset called TwiBot-20-Sub. In this way, the size
and label sparsity of the TwiBot-20-Sub dataset becomes comparable with the TIMME dataset.

5.1.3. Train-test Split
We split the users of both datasets into an 80%:10%:10% ratio for the training set, validation set, and
test set respectively.

5.2. Implementation Detail
To test the effectiveness of our framework across different models, we choose two single-modality text
encoders, GloVe and BERT, and two single-modality graph encoders, MLP and R-GCN.
   The GloVe embedding refers to the Wikipedia 2014 + Gigaword 5 (300d) pre-trained version. 2 The
BERT embedding refers to the sentence level ([CLS] token) embedding of BERT-base model [50] after
fine-tuning the pre-trained model’s parameters on the tweets from our training set consisting of 80%
of the users. We chose a max sequence length of 32. After the encoding, we have a 300-dimension text
embedding for GloVe and a 768-dimension text embedding for BERT.
   We choose a modified version of R-GCN from TIMME [21] as an R-GCN graph encoder. R-GCN
[34] is a GNN model specifically designed for heterogeneous graphs with multiple relations. In the
TIMME paper, it is discovered that assigning different attention weights to the relation heads of the
R-GCN model could improve its performance. Hence, we adopt their idea and use the modified version
of R-GCN. We did not use the complete TIMME model since it is designed for multiple tasks outside
our research scope, and will overly complicate our model.
   We also choose a 3-layer MLP as another graph encoder for comparison, the adjacency list for each
user is passed to the MLP.
   Large language models (LLMs) like ChatGPT are powerful in understanding texts, but they usually
have a great number of parameters, making traditional supervised fine-tuning a hard and costly task
[38]. Instead, less resource-intensive methods like few-shot learning, prompt tuning, instruction tuning,
and chain-of-thought are more frequently used to adapt LLMs on specific tasks [51]. We do not use
large language models as one of the options for the text encoder since those methods are not compatible
with our framework – they do not provide a well-defined gradient to train our attention gate learner.
   We run experiments on a single NVIDIA Tesla A100 GPU. We used the same set of hyper-parameters
as in the TIMME paper, with the learning rate being 0.01, the number of GCN hidden units being 100,
and the dropout rate being 0.1, on a PyTorch platform. For a fair comparison, we run over 10 random
seeds for each algorithm on each task.


6. Results and Analysis
6.1. Contribution Map
To show that our framework could effectively provide personalized explanations during the fuse of
modalities, we draw the contribution map based on ↵ (graph weight) and (text weight) attention for
users from each dataset. The darker the color, the weight of the corresponding modality is closer to 1.

2
    glove.6B.zip from https://nlp.stanford.edu/projects/glove/
In the contribution map, pure white indicates a zero contribution (0) from a modality, while pure dark
blue indicates a full contribution (1).
   The top figure of Figure 3 shows the contribution map output when the text encoder is BERT and the
graph encoder is R-GCN, on a subgroup of the TIMME dataset consisting of some politicians and some
random Twitter users. As we can see, there is a clear cut between the percentage of contributions from
different modalities to the final prediction. It is notable that for the two ambiguous politician users we
have mentioned earlier (Ryan Costello and Sheldon Whitehouse), CAMUE could give correct attention,
where we should trust more text data from Mr. Costello while trusting more graph structure data from
Mr. Whitehouse. To avoid any misuse of personal data information, we hide the names of random
Twitter users and only include politicians whose Twitter accounts are publically available at 3 .
   The bottom figure of Figure 3 shows the contribution map output when the text encoder is GloVe
and the graph encoder is R-GCN, on the same subgroup of the TIMME dataset. Note that for all shown
users text information does not contribute to the final prediction. This could be attributed to the
fact that GloVe is not very powerful for sentence embedding, especially when the text is long. This
contribution map shows that our framework filters out the text modality almost completely when it is
not helpful for our user embedding learning. As we can see from table 2, the traditional fusion method
for GloVe+R-GCN only yields an accuracy of 0.840, which is much lower than the single graph structure
modality prediction (0.953) using R-GCN, due to unreliable GloVe embedding. In contrast, our CAMUE
method obtains a higher accuracy (0.954) than all single modality models, since it disregards unreliable
information.
   Figure 4 shows the contribution map output for the same set of encoders on a subgroup of the
Twibot-20-Sub dataset. There is also a clear cut between the percentage of contributions from different
modalities, for both the human Twitter accounts and bot accounts.
   Hence, we verify that our framework could both provide personalized modality contribution and
drop low-quality information during the fuse of modalities. Some quantitative analysis of how this
low-quality information filtering could benefit the general model performance could be found in the
next section, and some qualitative analysis about what new insights we could gain from the output of
our framework could be found in the case study section.

6.2. General Performance
Table 2 shows the performance of CAMUE on different combinations of encoders. The traditional
fusion method in Figure 2 is denoted as “simple fusion”. For MLP, we do not have such a natural fusion
method. We also add “CAMUE, fixed params” as an ablation experiment to prove the effectiveness of
our attention gate-based selection module.
   We observe that within those combinations, sometimes simple fusion methods are significantly worse
than single-modality methods (e.g. GloVe+R-GCN vs R-GCN only) due to some untrustworthiness in
one of the modalities. However, any fusion under our CAMUE framework always performs better
than the corresponding single modality methods. That suggests that our algorithm can benefit from
attending to the more reliable modality between text and graph structure, if one particular modality is
not trustworthy (e.g. GloVe embedding), and learning not to consider it when making predictions (as
we can see in Figure 3, bottom).
   It is also notable that our CAMUE method outperforms “CAMUE, fixed params”. These results suggest
that adjusting the weight of different modalities dynamically yields better performance than fixed
weights of modalities. Finally, when the text modality is switched to a more accurate BERT embedding,
our framework still gives comparable performance to its corresponding simple fusion methods.

6.3. Case Studies
User Sub-groups Table 3 gives a quantitative analysis when the text encoder is BERT and the graph
encoder is R-GCN, for different sub-groups of Twitter users we are interested in. In general, graph
3
    https://tweeterid.com/
Table 2
The overall performance (format: accuracy ; f1-score) of CAMUE on different social media data sets.
                                      Encoder Variant                Data Set
                       Algorithm
                                       Text   Graph        TIMME         TwiBot-20-Sub
                                      GloVe              0.688 ; 0.681    0.565 ; 0.511
                        text-only             N/A
                                      BERT               0.862 ; 0.859    0.731 ; 0.722
                                              MLP        0.932 ; 0.930    0.707 ; 0.697
                        link-only     N/A
                                              R-GCN      0.953 ; 0.953    0.735 ; 0.728
                                      GloVe R-GCN        0.840 ; 0.837    0.683 ; 0.675
                     simple fusion
                                      BERT R-GCN         0.959 ; 0.959    0.791 ; 0.787
                                              MLP        0.938 ; 0.937    0.700 ; 0.691
                                      GloVe
                       CAMUE w.               R-GCN      0.952 ; 0.951    0.734 ; 0.727
                      fixed params            MLP        0.940 ; 0.938    0.732 ; 0.722
                                      BERT
                                              R-GCN      0.952 ; 0.951    0.779 ; 0.771
                                              MLP        0.945 ; 0.944    0.707 ; 0.697
                                      GloVe
                                              R-GCN      0.954 ; 0.953    0.738 ; 0.731
                        CAMUE
                                              MLP        0.935 ; 0.933    0.744 ; 0.738
                                      BERT
                                              R-GCN      0.961 ; 0.960    0.782 ; 0.776




Figure 3: Contribution map for TIMME dataset, top: CAMUE(BERT, R-GCN), bottom: CAMUE(GloVe, R-GCN),
dark blue stands for higher contribution while white stands for lower contribution, ranging from 0.0 to 1.0.




Figure 4: Contribution map for TwiBot-20-Sub dataset, CAMUE(BERT, R-GCN), dark blue stands for higher
contribution while white stands for lower contribution, ranging from 0.0 to 1.0.


structure information contributes the most when comes to bot accounts. One possible explanation for
this is the variety of bot accounts on Twitter, such as those for business advertising, political outreach,
and sports marketing [22]. Bots with different usage might talk very differently, however, they may
share some common rule-based policies when interacting with humans on Twitter [52, 53].
   Graph structure information contributes the second highest when it comes to politicians. This is also
not surprising since politicians are generally more inclined to retweet or mention events related to their
political parties [21]. It is also notable that the weight of text information for Republicans is slightly
less than that for Democrats. This aligns with the findings in [54] that Democrats have a slightly more
politically polarized word choice than Republicans.
   For random users, the weight of text information is the largest, although still not as large as the
weight of graph structure information. This could be attributed to the pattern that many random users
interact frequently with their non-celebrity families and friends on Twitter, who are more likely to be
politically neutral.

Table 3
% Users whose ↵ >          for different subgroups
                                                 Subgroup                    % Users
                                                Democrats                     70.9
                                                Republicans                   76.1
                                                 Politicians                  76.2
                                   Non-politicians with Party affiliations    72.4
                                         Non-bot random users                 61.2
                                               Bot accounts                   77.3
                                           TIMME, aggregated                  73.5
                                       TwiBot-20-Sub, aggregated              70.1


Table 4
Predicted Political Stance of Some News Agencies
                                     News Agency         Prediction    Text or Graph
                                    New York Times           D            Graph
                                   Washington Post           D            Graph
                                   Wall Street Journal       R              Text
                                      USA Today              D            Graph
                                          CNN                D            Graph
                                       Fox News              R              Text
                                       Guardian              D              Text
                                    Associated Press         R            Graph
                                        US News              D            Graph
                                        MSNBC                D            Graph
                                          BBC                R            Graph
                                    National Review          R            Graph
                                      Bloomberg              D             graph

  Table 4 shows some predicted political stances and the main contributing modalities of a group of
news agencies. We can see that most of them have more reliable information about graph structure
than text information. This is not surprising since most news agency tends to use neutral words to
increase their credibility, hence it is hard to gather strong political stances from their text embedding,
except for some of them like Fox News and Guardian which are known to use political polarized terms
more often [54, 55]. Our framework is able to capture this unique behavior pattern for Fox News and
Guardian, meanwhile giving mostly accurate political polarity predictions aligning with results in [54]
and 4 .
  To conclude, we are able to obtain customized user behavior patterns through our multi-modality
fusion. Those patterns could provide insights on which modality we should focus on more for different
types of users, for downstream tasks such as personalized recommendations, social science analysis, or
malicious user detection.

Selected Celebrities from TIMME Dataset Since there exists no ground truth contribution
4
    https://www.allsides.com/media-bias/ratings
Figure 5: A subset of the top 50 followed celebrities. Images licensed under CC-BY or sourced from publicly
available Twitter profile images for research purposes under the fair use term from Twitter’s Terms of Service.
Note that the political polarity prediction comes from our model and may not reflect their actual stances.


of the two aspects of user profiles (text and graph structure) on their final predictions, we do case study
by evaluating a subset of users qualitatively to validate our frameworks’ capability to give personalized
explanations. We selected 9 celebrities among the top 50 most followed Twitter accounts from 5 , whose
Twitter accounts appear in the TIMME dataset, as we are not allowed to disclose regular Twitter users’
information. We obtain the political polarity predictions of those celebrities and record the percentage
of text/graph structure information that contributes to their political polarity predictions (See Figure 5).
    • Elon Musk: Before 2020 (dataset cut-off), Elon Musk’s political views in his tweet text content are
      often complex. He claimed multiple times not to take the viewpoints in his tweets too seriously
      6 . This aligns with the low contribution weight of his texts on his political stand prediction.

      However, on the graph level, 66.67% of the politicians Elon Musk liked more than one time, have
      also liked Trump at least once. This is significantly larger than the average number in the TIMME
      dataset (23.67%). This could be a strong reason why our graph structure weight is so high and
      why we predict Elon Musk to be Republican-leaning. Our prediction is proved correct when in
      2022 (which is beyond our dataset cut-off time, 2020 [54]), Elon Musk claimed that he would vote
      for Republicans in his tweet 7 . This is a strong indicator that our framework is using correct
      information.
    • LeBron James: In his tweets, LeBron James frequently shows his love and respect to Democratic
      President Obama 8 . Our prediction for him to be Democrat-leaning with a strong text contribution
      aligns with this observation.
    • Lady Gaga: Similarly to James, Lady Gaga also expresses explicitly in her tweets about her support
      of Democratic candidates 9 . Our graph weight is 0, meaning that the text alone is sufficient to
      predict that she is Democrat-learning.
    • Bill Gates: He usually avoids making explicit statements about whether he supports Democrats
      or Republicans in his tweets. Although our model predicts him as the Republican, the probability
      edge is very marginal (11%).
    • Oprah Winfrey: During the 2016 presidential campaign, she retweeted and mentioned her support
      for Democratic candidate Hillary Clinton frequently 10 , making the graph structure information
5
  https://socialblade.com/twitter/top/100
6
  https://twitter.com/elonmusk/status/1007780580396683267
7
  https://twitter.com/elonmusk/status/1526997132858822658
8
  https://twitter.com/KingJames/status/1290774046964101123, https://twitter.com/KingJames/status/1531837452591042561
9
  https://twitter.com/ladygaga/status/1325120729130528768
10
   https://twitter.com/Oprah/status/780588770726993920
       a strong indicator of her Democratic stance.
     • Jimmy Fallon: Jimmy Fallon has managed to maintain a sense of political neutrality in his
       tweets. His text contribution to the final prediction is 0. Even though the Twitter graph structure
       indicates that he is Democrat-leaning, we still do not know in real life whether he is a Democrat
       or Republican.
     • Katy Perry: Just like Oprah Winfrey, Katy Perry also interacted with and supported Hillary
       Clinton during the 2016 election, a reason why we predict her as Democrat-leaning from the
       graph structure. Although she supports some republican politicians in 2022 11 , that is beyond the
       dataset cutoff.
     • Justin Timberlake: Justin Timberlake has frequent positive interactions with President Obama 12
       and firmly supports Hillary Clinton in his tweets 13 , both suggesting that he is Democrat-learning.
       Our model assigns a similar weight to text and graph structure, suggesting that both contribute
       to that prediction equally.
     • Taylor Swift: In the case of Taylor Swift, the model fails to give the correct prediction. Her tweets
       show that she voted for Biden during 2020 14 , but the prediction is Republican. One reason is that
       at the graph structure level, the majority of Taylor Swift’s followers are classified as Republican
       (67.09%) in the dataset, which can mislead the graph encoder.

  Overall, we conclude that graph structure information is usually more useful when predicting the
political polarities of those celebrities. That aligns with the quantitative results in table 3. As we can
see, different celebrities may have very different behavior patterns. Those patterns can be correctly
captured and explained by our contribution weight. That confirms the effectiveness of our framework.


7. Conclusion
In this paper, we investigate some potential limitations of existing fusion methods for text information
and graph structure information in user representation learning on social networks. We then propose
a contribution-aware multimodal social-media user-embedding with a learnable attention module.
Our framework can automatically determine the reliability of text and graph-structure information
when learning user-embeddings. It filters out unreliable modalities for specific users across various
downstream tasks. Since our framework is not bound to any specific model, it has great potential to be
adapted to any graph-structure-embedding component and text-embedding component, if affordable.
More importantly, our models can give a score on the reliability of different information modalities
for each user. That gives our framework great capability for personalized downstream analysis and
recommendation. Our work can bring research attention to identifying and removing misleading
information modality due to differences in social network user behavior, and paves the way for more
explainable, reliable, and effective social media user representation learning.
   Some possible future extensions include adding more modalities other than text and graphs (e.g.,
image and video data from user’s posts). Also, we consider the user identities to be static throughout
our analysis, which might not be the case in many scenarios. We can bring time as a factor to produce a
multi-modality dynamic social media user embedding. For example, a user’s text content may be more
trustworthy in the first few months, and interactive graph structure information becomes more reliable
in longer terms.




11
   https://twitter.com/katyperry/status/1533246681910628352
12
   https://twitter.com/jtimberlake/status/1025867320407846912
13
   https://twitter.com/jtimberlake/status/768191007036891136
14
   https://twitter.com/taylorswift13/status/1266392274549776387
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