[Extract] 1. Introduction The so-called Web 2.0 brought a new breadth to the Internet, and a social perspective that seems set to stay. Services such as LinkedIn, Hi5, and Facebook have found a place in our society. People connect to each other through common paths. Meta-APIs such as Google's November 2007 release, OpenSocial, enable social applications to operate across multiple sites and services, providing a way to relate much of this data. In social bookmarking tools (e.g. Del.icio.us, Connotea, Bibsonomy), and media sharing services (such as Youtube, Flickr, Picasa, Slideshare) people are asked to tag and otherwise annotate and share their resources inside communities or at a global scale, creating a huge amount of user generated metadata (tags) with a clear value for information discovery...