Social bookmarking is an emerging type of a Web service for reusing, sharing, and discovering resources. By bookmarking, users preserve access points to encountered documents for their future access. On the other hand, the social aspect of bookmarking results from the visibility of bookmarks to other users helping them to discover new, potentially interesting resources. In addition, social bookmarking systems allow for better estimation of the popularity and relevance of documents. In this chapter, we provide an overview of major aspects involved with social bookmarking and investigate their potential for enhancing Web search and for building novel applications. We make a comparative analysis of two popularity measures of Web pages, PageRank and SBRank, where SBRank is defined as an aggregate number of bookmarks that a given page accumulates in a selected social bookmarking system. The results of this analysis reveal the advantages of SBRank when compared to PageRank measure and provide the foundations for utilizing social bookmarking information in order to enhance and improve search in the Web. In the second part of the chapter, we describe an application that combines SBRank and PageRank measures in order to rerank results delivered by Web search engines and that offers several complimentary functions for realizing more effective search.
TopIntroduction
Social bookmarking is one of the main trends of a new generation of the Web called Web 2.0. The idea behind social bookmarking is to let users store URLs to their favorite pages and make them visible to other users. Each social bookmark is annotated with tags that describe the bookmarked resource and that were freely chosen by a bookmarker. Del.icio.us1 is currently the most popular social bookmarking service. It has been operating since 2003 and currently has about 3 million users that bookmarked around 100 million Web documents2. There are also other popular social bookmarking sites such as Furl3 or Simpy4.
Non-social bookmarking was proposed first by (Keller, Wolfe, Chen, Labinowitz, & Mathe, 1997) as a way to remember and locally store access points to visited Web documents. In social bookmarking the social aspect of bookmarking allows for discovery of new, potentially relevant resources thanks to the combined effort of many users. This makes it also possible to determine the resources that are both relevant (by the analysis of their tags) and recently popular (by counting their bookmarks) as well as permits to track their popularity and relevance over time. For example, del.icio.us informs users about popular pages that recently obtained many bookmarks and cloudacio.us5 displays historical patterns and trends of bookmarked resources.
The incentives of social bookmarkers have been recently categorized by (Marlow, Naaman, Boyd, & Davis, 2006). According to the authors, users decide to bookmark the resources because of the following reasons: future retrieval, contribution and sharing, attract attention, play and competition, self presentation, opinion expression. In most cases, however, bookmarking are useful for individual users who want to externally (hence beyond the limit of a single PC machine) store access points to their selected resources. However, in this way, the users help also to manually arrange the Web in a bottom-up fashion since they categorize the online resources and enable better estimation of their popularity and, indirectly, quality.
An important characteristic of tagging in social bookmarking systems is the lack of any controlled vocabulary. Users are free to annotate bookmarked documents as they wish or they can borrow same tags as others used. However, after some time, certain forms of tag agreements emerge for the resources as demonstrated in (Golder & Huberman, 2006). The process of resource categorization by free tagging is called folksonomy and is inherently different from a rigid classification usually done by domain experts, for example, by librarians. However, the well-known problems with folksonomy result from its advantages, that is, from the uncontrolled, free character of categorizing resources by many users. For example, ambiguity, synonymy or polysemy occur among tags that can undermine the retrieval process.
In this chapter we discuss the social bookmarking phenomenon and provide the results of analytical study aimed at analyzing the usefulness of social bookmarks for improving the search in the Web. In particular, we perform a comparative analysis of two popularity measures of Web pages, SBRank and PageRank. PageRank is a popular iterative algorithm that scores Web pages based on the random surfer model (Page, Brin, Motwani, & Winograd, 1998). In short, a page has a high PageRank value if it is linked from a relatively large number of other pages that also have high PageRank scores. By finding popular resources both SBRank and PageRank provide means for selecting high quality Web pages assuming a positive correlation between popularity and quality. In addition, we analyze several other aspects of pages bookmarked in social bookmarking sites. For example, we investigate the dynamics of the both metrics in order to confirm whether mixing them could improve the dynamic characteristics of search results.