I should add that the trickiest bits aren't the apps that could "run on top of" a distributed social network like photo tagging or games. It's setting up the so

I should add that the trickiest bits aren't the apps that could "run on top of" a distributed social network like photo tagging or games. It's setting up the social graph those apps use. How do you add a friend? How do you accept or reject a friend request? What sort of authentication and other security features should everyone understand and support? There are proposals for solving these issues but getting it a) secure and b) widely agreed upon and supported would be a Good Thing.