Elgg is an award winning open source social networking platform. It can be used for building different types of social networking environments for universities, schools or colleges, or running an internal collaborative platform for your organization. Elgg is a feature rich platform that can help you easily build your project. Some of the highlighted features are the powerful data model, activity streams, user management, access control and others. You can browse the Elgg plugin gallery for even more features:
If you want to build a community website you should definitely give Elgg a try!