Pleroma : Pleroma is a federated social networking platform, compatible with GNU social and other OStatus implementations. It is free software licensed under th...
pump.io : Social server with an ActivityStreams API. It's a stream server that does most of what people really want from a social network.
gollum : A simple, Git-powered wiki with a sweet API and local frontend.
Shariff : Shariff enables website users to share their favorite content without compromising their privacy.
GNU social is a continuation of the StatusNet project. It is social communication software for both public and private communications. It is widely supported and has a large userbase. It is already used by the Free Software Foundation.