I've been thinking of something along this very line, but I'm a little torn: distributed is the right way to go, but to implement something akin to the facebook newsfeed, it seems like the right answer is an atom feed that all your friends subscribe to (and you to theirs), but then either you have something downloading theirs all the time, and then your info is stored on someone else's computer where it's easy pickings for a bot (and the opportunities for exposure multiplied), or you wait to fetch all 100+ friends' feeds. I'm not sure if the risk of the former is *that* big, after all, it wouldn't be hard for a bot to get the facebook login and skim all the info, but it would be rather harder than just picking off the local cache.
Building an distributed network on top of HTTP protocols (atom) will lead to privacy problems... It seems necessary to have a server-side support that will also implement some privacy policies which would allow you to define who gets access to which post. Also, the only way to get real time updates is by some polling, which may be resource intensive...
Using XMPP (Jabber) as a base for some kind of distributed social network seems to me like a better idea to me... It is still distributed, you can run your own server that will host your posts and will push all the updates to those subscribed, implement some privacy rules...
There is already a project that tries to do something like that: http://onesocialweb.org/
