There's OneSocialWeb http://bit.ly/OneSocialWeb with a great engineering team (talented chaps like @eschnou) and backing of deep pocketed Vodafone. Open sourced under an Apache 2 license.
Also there's the free open source distributed/federated www.ThinkUpApp.com from Gina Trapani and Anil Dash which can probably be built out into something along the lines of what you are thinking.