section title headers Table 1. Data from four social bookmarking services, compared with our CiteULike data Name Data collection Del icio us" Collaborative tagging system for Web bookmarks Four days(212 URLs; 19, 422 bookmarks) Flickr Photo-sharing system for users to store and tag their and others' personal photos No time data (25,000 users) Social bookmarking service for a large enterprise(IBM's intranet) Eight weeks(13, 174 bookmarks, 686 users) Movielens Movie recommender system that also lets users tag their favorite movies Approximately one month(3,26 tags: 635 users) CiteULike Social bookmarking service for sharing, storing, and organizing scholarly papers More than two years(2,0Il users: 9,623 papers: 6, 527 tags) Social Bookmarking Anatomy The basic unit of information in a social book. marking service comprises three elements in a triple, represented as(user, resource, tag) User Adapting terminology from previous work,this riple is called a tag application (in which a user applies a tag to a resource; in some cases, it's also called a tag post). The combination of elements in a tag application is unique- that is, if a user tags a paper twice with the same tag, it counts as only one tag application. Figure 1. Anatomy of a tag application in CiteULike User /(blue) Resources can mean different things for differ- has two applications and user 2 (yellow) has one tag application ent social bookmarking services. With delicio us, for example, the resource is a Web site; with CiteU- Like, it's a scholarly paper. challenges--Puturn possibilit Adapting social bookmarking,'s schematic depic- tion from Ciro Cattuto's work, Figure 1 illustrates the schema for tag applications in CiteULike. This example has three tag applications: (user 1. paper bc."tag“A"),(user1, paper“xyz",tag We analyze CiteULike's data around the (user resource, tag) elements in the tag application We compare and contrast our results, in general with four other social bookmarking analyses. Table 1 briefly lists each one's purpose and how much data the researchers collected while analyz ing that service; we also include the CiteULike data set we analyze in this article CiteULike overview CiteULike is a free online social bookmarking serv ice that lets researchers share, store, and organize nformation about scholarly papers. Users can add links to papers on CiteULike to their own online (b) collections and import references from other schol- arly digital libraries(Figure 2a). For example, users Figure 2. The CiteULike social bookmarking service. ( a)A screenshot can link to an IEEE or Cite Seer paper in their per- of the Web site shows a scholarly paper tagged in CiteULike. b The sonal CiteULike collection. The service also provides tagging page on CiteULike additional information about the paper, such as all users'tags for that paper and the BibTex entry Adding papers to a personal collection and tag- view the link to a favorite paper, they see every- ging them is a two-stem process When users first one's tags for that paper(Figure 2a).However, to NOVEMBER. DECEMBER 2007Social Bookmarking Anatomy The basic unit of information in a social book￾marking service comprises three elements in a triple, represented as (user, resource, tag). 2 Adapting terminology from previous work, 3 this triple is called a tag application (in which a user applies a tag to a resource; in some cases, it’s also called a tag post). The combination of elements in a tag application is unique — that is, if a user tags a paper twice with the same tag, it counts as only one tag application. Resources can mean different things for differ￾ent social bookmarking services. With del.icio.us, for example, the resource is a Web site; with CiteU￾Like, it’s a scholarly paper. Adapting social bookmarking’s schematic depic￾tion from Ciro Cattuto’s work, 2 Figure 1 illustrates the schema for tag applications in CiteULike. This example has three tag applications: (user 1, paper “abc,” tag “A”), (user 1, paper “xyz”, tag “A”), and (user 2, paper “abc,” tag “B”). We analyze CiteULike’s data around the (user, resource, tag) elements in the tag application. We compare and contrast our results, in general, with four other social bookmarking analyses. 3–6 Table 1 briefly lists each one’s purpose and how much data the researchers collected while analyz￾ing that service; we also include the CiteULike data set we analyze in this article. CiteULike Overview CiteULike is a free online social bookmarking serv￾ice that lets researchers share, store, and organize information about scholarly papers. Users can add links to papers on CiteULike to their own online collections and import references from other schol￾arly digital libraries (Figure 2a). For example, users can link to an IEEE or CiteSeer paper in their per￾sonal CiteULike collection. The service also provides additional information about the paper, such as all users’ tags for that paper and the BibTeX entry. Adding papers to a personal collection and tag￾ging them is a two-stem process. When users first view the link to a favorite paper, they see every￾one’s tags for that paper (Figure 2a). However, to NOVEMBER • DECEMBER 2007 17 section title headers Figure 1.Anatomy of a tag application in CiteULike.User 1 (blue) has two applications and user 2 (yellow) has one tag application. Paper abc Paper xyz Tag A Tag B User 1 User 2 Table 1. Data from four social bookmarking services, compared with our CiteULike data. Name Purpose Data collection Del.icio.us4 Collaborative tagging system for Web bookmarks Four days (212 URLs; 19,422 bookmarks) Flickr5 Photo-sharing system for users to store and tag their and others’ personal photos No time data (25,000 users) Dogear6 Social bookmarking service for a large enterprise (IBM’s intranet) Eight weeks (13,174 bookmarks; 686 users) MovieLens3 Movie recommender system that also lets users tag their favorite movies Approximately one month (3,263 tags; 635 users) CiteULike Social bookmarking service for sharing, storing, and organizing scholarly papers More than two years (2,011 users;9,623 papers;6,527 tags) Figure 2.The CiteULike social bookmarking service. (a) A screenshot of the Web site shows a scholarly paper tagged in CiteULike. (b) The tagging page on CiteULike. (a) (b)