Social media sites can act as a rich source of large amounts of data by letting anyone easily create content on the Web. The SIOC ontology and tools developed for it allow us to express this information as interlinked RDF data. This paper describes approaches for making this Social Web information an integral part of the Web of Linked Data.
SIOC (Semantically Interlinked Online Communities) [
This paper describes how Social Web data, described in SIOC, can be further weaved together with other kinds of linked data. We describe the structure of SIOC data and give a description of different approaches to interlink SIOC data with other sources of linked data. Finally, we conclude the paper with some interesting uses of such data.
The SIOC implementations list3 contains around 35
applications for creating and using SIOC data. By installing
relevant SIOC export plugins, online community sites can
generate linked data and start forming a critical mass of
RDF data about user-created content in the same way as
LiveJournal did for FOAF data. Other tools allow users
to browse SIOC data or to translate existing data, such as
mailing list archives, to SIOC. Moreover, SIOC is also used
for enterprise data integration[
.... .... .... .... .... ....
• a blog profile links to all posts on the blog; • posts link to replies / comments; • posts and comments link to the profile of a user who created them (if a user is registered on a site).
This makes SIOC-enabled social media sites a rich source of
interlinked data and enables an RDF crawler to start at the
main profile of a site and retrieve all SIOC RDF pages that
this site provides from a single entry point, i.e. the RDF file
describing the sioc:Site. Almost all SIOC-enabled sites
also have RDF autodiscovery links in their web pages,
making it easier to find RDF data for people and applications
when opening a page. Another interesting use of this
autodiscovery feature is implemented in the Semantic Radar
Firefox extension6. The extension automatically sends a
notification to the Ping The Semantic Web service7 whenever
the user accesses an autodiscovery-enabled web page. This
allows people to participate in the distributed discovery of
Semantic Web content [
While these features makes a SIOC-enabled site a good source of linked data, at this point it can still be viewed as a “walled garden” and not really as an integral part of a larger Web of Linked Data. On its own, SIOC data from a single exporter may have a limited connectivity to the “outside world”. Links to other websites are mainly achieved at the moment thanks to the extraction of HTML hyperlinks from post content. These hyperlinks are republished in the post’s SIOC profile using the sioc:links_to property.
There are two paths to making online community data, described in SIOC, a more integrated part of the Web of Data: linking into “SIOC space”, and referencing linked data from inside online community sites. We will now describe both of these paths, starting with linking to SIOC data.
The question of receiving links from the “outside world”, strictly speaking, is not something SIOC exporters can directly influence as this depends on other sources of data
linking to SIOC Sites(s). Yet, there are some things that can be done to facilitate linking to SIOC: • owners of FOAF profiles can link to social media sites and their user accounts on these sites; • SIOC exporters can be optimized to make SIOC data easier to discover; • Semantic Web indexing and lookup services can find and provide access to SIOC data.
A simple and effective way to link to SIOC profiles is to point to them from personal FOAF profiles. For example, people often already link to their blogs from FOAF using the foaf:weblog property. In this case they just need to add an rdfs:seeAlso link pointing to the weblog’s SIOC profile, which also mentions the weblog’s homepage URI and thus allows a client to connect the information in the two documents.
People can also use the foaf:holdsOnlineAccount property to point to user accounts (sioc:User) that they have registered on online community sites (Fig. 2). As a part of SIOC data, these user accounts are normally linked to content items created by the user.
If we can make it easier for applications and users browsing the Web to find SIOC data, they are more likely to use this data. RDF autodiscovery links play an important role in helping one to find Semantic Web data, and almost all SIOC exporters use them, making it a reliable way to detect the presence of SIOC data. As a result, all web pages on a SIOCenabled site contain RDF autodiscovery links to relevant RDF data (e.g., a blog post will point to data about this particular post).
Another way of improving discoverability is content
negotiation. The idea is that client requests to URIs of resources
such as forum posts are answered with either an HTML or
an RDF representation, depending on the client’s
preferences. This lets existing URIs of the forum’s web pages,
which are already used as targets of HTML links, double as
access points to the SIOC data when used by RDF-aware
clients. We have experimented with adding content
negotiation to some SIOC exporters, but this has proven to be
challenging. First, implementing full content negotiation as
described in the HTTP specification is fairly complex.
Second, it is hard to verify that an implementation works
correctly with the large number of different versions of Web
and data browsers in use on the Web, some of which express
questionable preferences. Third, the plugin systems of some
content management systems do not provide the necessary
hooks for implementing content negotiation. Therefore, we
recommend that linked data browsers such as Tabulator [
SIOC exporters can also use RDFa8 to embed SIOC data directly in HTML documents, e.g. blog posts. A first approach to embed RDFa in Drupal has been explored9, and future work may go in this direction, since it provides another way to discover RDF triples related to a document. Finally, some site administrators have decorated their pages with small SIOC icons that link directly to the related SIOC profile. These icons can be considered redundant as we already have RDF autodiscovery links, or can even be considered harmful as users who click the icon in non-RDF-aware Web browsers will be subjected to raw RDF/XML code. On the other hand, such icons are a useful marketing tool that increases awareness of SIOC in general.
In general, it can be said that we desire a user experience where URIs of Web resources such as blog posts can simply be used in HTML hyperlinks and RDF statements to refer to the resource. Mechanisms such as RDF autodiscovery, content negotiation or RDFa parsing should be transparent to the user and entirely reliable.
An important aspect of linked data is being able to find
what data is linking into a resource. Online community
sites sometimes notify each other of links they make by
using pingbacks or trackbacks, but other publishers usually do
not send such notifications. This is where Semantic Web
indexing services such as Sindice [
Some resources that users may want to link to include: • Persons - authors of posts and comments => link to their FOAF URI and FOAF profile; • Topics - categories and tags => link to other linked data about these topics;
thread/thread/b1585e9ef3a17665 • Other data - information associated with / embedded within content.
FOAF is one of the most successful Semantic Web vocabularies and is often used to describe personal information and social network relations. One of the first use cases for linking to other RDF data is to link posts or comments to FOAF profiles of their creators. Currently sioc:Post(s) are linked to a creator’s user profile on a community site, but export tools can be easily extended to point to a provided FOAF profile and to the URI of a creator of this content. In order to point to FOAF profiles, the application has to know what URI to point to. Users of a community site (e.g., post authors) can supply this information when registering on the site, but comment authors may not be motivated enough to provide the necessary FOAF information and an automated process for finding this data is preferable. This can be achieved automatically using OpenID10: A user authenticates using OpenID and a blog engine checks an OpenID URI for links to person’s FOAF profile. If such a link is found, statements pointing to the person’s FOAF profile (e.g., owl:sameAs and rdfs:seeAlso) can be added to SIOC metadata describing the author of the comment. Some work regarding this point are currently done within the SparqlPress11 project, which aims to provide Semantic Web functionalities, in both exporting, crawling and linking RDF data.
Categories and tags which describe the topic of the content
are also good candidates for pointing to additional
information. SIOC exporters use a sioc:topic property to point to
a topic URI. Topic category hierarchies can be described in
SKOS and topics can be linked to more information about
them such as RDF data from DBpedia [
One option is to add this knowledge to the site in advance.
Adding annotations every time a new topic is created may
work for categories that do not change very often, but is not
feasible for tags which are more dynamic. Another option is
to rely on the MOAT framework, which allows us to assign
meaning to tags in a collaborative way using existing URIs
from datasets such as DBpedia or GeoNames [
tagging/1 tags:taggedBy tags:taggedResource ht p:/ example.org/alex ht p:/ example.org/ post/1 sparql moat:tagMeaning
There are other resources that authors may want to describe or link to. Some SIOC exporters have built-in functionality for defining links to additional RDF data. The Drupal and WordPress exporters pass post content through a filter to find hyperlinks with MIME type application/rdf+xml. Appropriate rdfs:seeAlso statements are added to the generated SIOC RDF data for this post in addition to the usual sioc:links_to property. This simple mechanism allows us to add links to RDF data to existing blog posts with very little effort. As a possible extension of this mechanism, some information about the linked resources may be copied into the post’s SIOC profile, e.g. rdfs:label and rdf:type information which assists users of data browsers in navigation. Sometimes, having a link to external RDF data is not enough. Within a post, a user may want to express some additional machine-readable information about the objects discussed in the post. As example use case an author describes a software project, points to DOAP (Description Of A Project) data describing this project, and includes a review of this project. This information can be added to SIOC data, but we need a way to add additional information to content items and to be able to retrieve it later.
Such embedded annotations are not widespread yet and current SIOC tools do not implement this functionality, but they can be extended to support such a use case. Filters for extracting metadata from post content can be executed one after another and RDF data can be extracted and added to the generated SIOC data. Data can be embedded in content in a number of different ways – RDFa, GRDDL13, microformats – as long as appropriate content extraction modules are available.
The Social Web is not limited to forums or blogs. There are
different kinds of social media and Web 2.0 sites, such as
Flickr, Twitter and Facebook, which offer interesting
content that can be described in RDF using FOAF, SIOC and
13http://www.w3.org/TR/grddl-primer/
related vocabularies [
Information from users’ contribution to these sites can be interesting at different levels. There are social networks, which are formed by users on different sites. These networks can be described in FOAF. Web users may also have a FOAF profile which points to their different accounts on different sites. Users create content items (sioc:Item) such as videos, bookmarks, etc. which can be organised in containers (Fig. 4). When expressed in RDF, this information forms an interlinked web of rich social data, ready for reuse. Moreover, the use of the SIOC types module15 allow people to decribe exactly the type of their content (eg: sioct:BlogPost, sioct:VideoChannel).
Two interesting and emerging applications of such
information are object-centred sociality and social media
portability. Object-centred sociality looks at objects such as content
items which people create and co-annotate as a medium
through which people are connected together. The use of
SIOC data for object-centred sociality is explored in [
SIOC data created by online community sites are highly interlinked and ready for weaving into a larger Web of Data. In this paper we described some approaches for facilitating the linking to SIOC data and for using SIOC to link back to other RDF data. 14http://sioc-project.org/node/262 15http://rdfs.org/sioc/types 16http://www.dataportability.org/ While some may consider social media content just an ”endpoint” in a journey for exploration of linked data, interesting possiblities arise when these content items are both linked to and contain links to other RDF data. In this case social media content and associated SIOC data act as a linking point by connecting together different parts of the linked data universe.
The authors are looking forward to feedback and suggestions from other implementers of Linked Data on the Web for enabling interoperability and reuse of the SIOC data and tools described here.