How would that look from an end-user standpoint? If I'm a person who doesn't know a thing about protocols or technology and uses Facebook a lot, what causes me to start to migrate over to the distributed services? Particularly because one of the great strengths of Facebook is exactly that it's not distributed: all the people you know, in one place. So it seems like to replace one of the core functionalities provided, there has to be some centralized component. And then it's hard for me to see what benefit decentralization is providing for either the user or the service.
An alternative social architecture seems appealing in some ways but I just can't see the [realistic, might actually happen with a significant chunk of the Facebook crowd] vision of how we'd ever get there from here.