Research on participatory websites has been minimally theoretical and lacks a comprehensive framework that identifies common elements and their functions across a variety of Web 2.0 platforms. This article suggests the definitions of 4 common message types in participatory websitesproprietor content, user-generated content, deliberate aggregate user representations, and incidental aggregate user representationsand offers research exemplars that illustrate how they may function in transforming online social interaction and influence. It introduces the 6 empirical studies in this Special Issue of the Journal of Computer-Mediated Communication in terms of the theories, functions, cues, and message types on which these articles focus.