This book constitutes the refereed proceedings of the Third International Conference on Online Communities and Social Computing, OCSC 2009, held in San Diego, CA, USA in July 2008 in the framework of the 13th International Conference on Human-Computer Interaction, HCII 2009 with 10 other thematically similar conferences. The 77 revised papers presented were carefully reviewed and selected from numerous submissions. The papers accepted for presentation thoroughly cover the thematic area of online communities and social ...