I'd say that RSS *is already* the basis for an open distributed version of twitter, facebook and so on.
So, what's missing here? Simple: someone please create a feed reader where I can also directly publish to my blog. Better yet, WordPress.org or any other blogging tool could have an embedded feed reader.
What would happen if we already had that? Well, I could be reading this post on my feed reader (aka distributed twitter) and then I could directly write a post on my blog replying to this post (aka distributed retweet or reply).