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.
freedom.js : freedom.js is a framework for building peer-to-peer (P2P) web apps. Easily create social applications that work in modern web browsers, Chrome package...
Elgg : Elgg is an award-winning social networking engine, delivering the building blocks that enable businesses, schools, universities and associations to cr...
librehosters : librehosters is a network of cooperation and solidarity that uses free software to encourage decentralisation through federation and distributed platf...
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.