The Web 2.0 is leading a new revolution on the World Wide Web, where social annotations have become more and more popular. On Web 2.0 sites, like Delicious and Flickr, various kinds of Web resources are popularly annotated by Web users with freely chosen keywords, which are also known as social annotations or tags. Such annotations not only benefit Web users, but also are helpful in many Web applications. For example, the social annotations of Web pages can be used to improve Web search [47, 735, 773], enterprise search [174], personalized search [503], Web browsing [400], and Semantic Web [729, 778]. Annotations given to other resources are also useful, for example, annotations of Flickr which are assigned to pictures can be effectively utilized for event visualization and detection [186].