Building user communities can contribute significantly to the success of an on-line platform. The literature on establishing on-line communities and on extending existing platforms with functionalities that enable community building emphasizes the social and economic aspects of community building and neglects the technical aspects. On-line communities have two elements: community members and platform. The platform provides a virtual social meeting space and shapes communication with the service it provides. In order to determine what kind of services will enable, stimulate, and foster community building, this paper identifies community modules and proposes a typology of on-line communities and their community-supporting platforms.