Here are some great resources for online communities:
Learning Communities are an important type of online community. Chapter 2 – Online Learning Communities: Common Ground and Critical Differences in Designing Technical Environments by Margaret Riel and Linda Polin (2004) covers several types of online learning communities (shared with permission).
Situated Learning: Legitimate Peripheral Participation is the original research on communities of practice by Jean Lave and Etienne Wenger from 1991.
Digital Habitats is more recent research from Etienne Wenger, John Smith and Nancy White applying the theory of community of practice to online communities.
Online Learning Circles are great for encouraging interaction and distributed leadership in online learning communities.
Power Laws, Weblogs and Inequality by Clay Shirky is Chapter 3 from Extreme Democracy. This article explains how a small number of participants makes the bulk of the contributions in online communities.