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19.1 Introduction With the advent of affordable domestic high-speed communication facilities, inexpensive digitization devices, and the open access nature of the Web, a new and exciting family of Web applications known as Web 2.0 has been born. The underlying idea is to decentralize and cheapen content creation, thus leading theWeb into a more open, connected, and democratic environment. In this chapter we will focus on a particular family of Web 2.0 applications known as Social Tagging Systems (STS for short). STS assign a major role to the ordinary user, who is not only allowed to publish and edit resources, but also and more importantly, to create and share lightweight metadata in the form of freely chosen keywords called tags. The exposure of users to both tags and resources creates a fundamental trigger for communication and sharing, thus lowering the barriers to cooperation and contributing to the creation of collaborative lightweight knowledge structures known as folksonomies1. Some notable examples of STS are sites like Delicious2, BibSonomy3, and Last.fm4, where Delicious allows the sharing of bookmarks, BibSonomy the sharing of bookmarks and lists of literature, and Last.fm the sharing of music. These systems are characterized by being easy to use and free to anyone willing to participate. Once a user is logged in, he can add a resource to the system, and assign arbitrary tags to it. If on the one hand this new family of applications brings new opportunities, it revives old problems on the other, namely the problem of information overload. Millions of individual users and independent providers are flooding STS with content and tags in an uncontrolled way, thereby lowering the potential for content retrieval and information sharing. One of the most successful approaches for increasing the level of relevant content over the “noise” that continuously grows as more and more content becomes available online lies on Recommender Systems (RS for short). In STS however, we face several new challenges. Users are interested in finding not only content, but also tags, and even other users. Moreover, while traditional RS usually operate over 2-way data arrays, folksonomy data is represented as a thirdorder tensor or a hypergraph with hyperedges denoting (user, resource, tag) triples. Furthermore, while there is an extensive literature for rating prediction based on explicit user feedback, i.e., a numerical value denoting the degree of preference of a user for a given item, in folksonomies there are usually no ratings. Thus, before arguing why not to simply use an old solution to a recurrent problem, we need to investigate to which extent the traditional RS paradigm and approaches apply to STS.Social tagging recommender systems is a young research area that has attracted significant attention recently, which is expressed by the increasing number of publications (e.g., [15, 11, 37, 35, 31]) and is poised for continued growth. Furthermore,
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