In a nutshell

The purpose of onesocialweb is to enable free, open, and decentralized social applications on the web. Its protocol can be used to turn any XMPP server into a full fledged social network, participating in the onesocialweb federation. The suite of extensions covers all the usual social networking use cases such as user profiles, relationships, activity streams and third party applications. In addition, it provides support for fine grained access control, realtime notification and collaboration.