A major challenge in fostering online communities is developing and sustaining communities. This paper integrates social capital and group development theories in order to derive a framework that seeks to explain why some online communities prosper, while others decline and perish. Four facets of social capital - network structure, identification, obligations, and shared vision - are proposed to moderate the relationship between online community development, developmental processes, and effectiveness. It is suggested that online communities who lack a common stock of social capital are more inclined to follow the stage model of group development. In contrast, online communities that have optimal levels of social capital are more likely to follow the punctuated equilibrium model of group development. The paper concludes with implications for theory and practice.