Tagging communities are popular instances of a broad class of online communities based on user-generated content. In these communities users introduce and tag content for later use. Although recent studies advocate and attempt to harness social knowledge generated in this context, little research has been done to quantify the current level of user collaboration in these communities. This paper introduces two metrics to quantify the level of collaboration: content reuse and shared interest. Using these two metrics, this paper shows that the current level of collaboration in CiteULike and Connotea is consistently low, which significantly limits the potential of harnessing the social knowledge in collaborative communities. This study also discusses implications of these findings in the context of recommendation and reputation systems.
