The recent activity stemming from Brad Fitzpatrick's Thoughts on the Social Graph reminds me of something I was thinking a lot about back in 2004. See especially Mark Atwood's post Five billion social network sites, each about one person on the Social Network Portability list.
The idea I had three years ago was the notion of people hosting their own data and the social networking sites merely being aggregators.
Actually, the way it occurred to me first was in the context of DOAP and the next Advogato but I then generalized the idea in Aggregation Versus Hosting.
Steve Mallett started a mailing list and a website about this same idea, calling it DataLibre. Unfortunately the site no longer exists but here is an announcement of the original site: http://www.oreillynet.com/ruby/blog/2004/09/datalibre_update.html
Steve really drove the vision but I posted numerous thoughts on the idea. Because my blog didn't have categories at the time and I haven't gone back and categorized my old stuff here's a list of my relevant posts:
Brad's article (which is a lot more concrete than anything I ever said) seems to be rejuvenating discussion about this sort of thing. See, for example, the post More on social network portability on Plaxo's blog.