Social Tagging Systems are web applications in which users upload
resources (e.g., bookmarks, videos, photos, etc.) and annotate it with a
list of freely chosen keywords called tags. This is a grassroots
approach to organize a site and help users to find the resources they
are interested in. Social tagging systems are open and inherently
social; features that have been proven to encourage participation.
However, with the large popularity of these systems and the increasing
amount of user-contributed content, information overload rapidly becomes
an issue. Recommender Systems are well known applications for increasing
the level of relevant content over the "noise" that continuously grows
as more and more content becomes available online. In social tagging
systems, however, we face new challenges. While in classic recommender
systems the mode of recommendation is basically the resource, in social
tagging systems there are three possible modes of recommendation: users,
resources, or tags. Therefore suitable methods that properly exploit the
different dimensions of social tagging systems data are needed. In this
book, we survey the most recent and state-of-the-art work about a whole
new generation of recommender systems built to serve social tagging
systems. The book is divided into self-contained chapters covering the
background material on social tagging systems and recommender systems to
the more advanced techniques like the ones based on tensor factorization
and graph-based models.