It'd be slow, though, like a lot of the darknet protocols, and would mean that whether or not your post is visible depends on which users you're connected to. I think the usability problems would likely rule it out.

P2P works great for filesharing because you're reading small numbers of large things, so if each thing takes a minute or two to start, that's fine. It would suck for reading the memes community on a social network cos you need to read many small things, and if they don't load quickly the experience is ruined.