A content addressable base architecture upon which to build interlocking, overlapping public/private information spheres is, in my opinion, one of the most critical infrastructure projects of our time.
Camlistore http://camlistore.org/ is the most advanced software of this genre that I have seen. I hope that it may serve as a foundation for a truly open and distributed network of "social" data such as "likes" and "circles", as well as and many other types of content nodes and relationships between them.
Brad Fitzpatrick, chief instigator of Calimstore recently described it thus:
Think of it as a universe where everything is content-addressable and everybody has their own personal, private repo of all their data (files, content, likes, bookmarks, whatever) and they can search their own stuff, or selectively re-share it out to friends. People can use hosted servers or run their own, an