"Tagging works because it strikes a balance between the individual and social. It serves the individual motive of remembering, and forms a ad-hoc social groups around it."
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Y. Song, L. Zhang, and C. Giles. CIKM '08: Proceeding of the 17th ACM conference on Information and knowledge mining, page 93--102. New York, NY, USA, ACM, (2008)