Bittorrent is different. You don't care who the other nodes are, all you care about is you're connected to some nodes, and the client takes care of that for you.
Like others said, the only way a decentralized facebook would work is when everyone uses one (or two) central nodes (site) which will most likely be run by something like Google or Microsoft, and the end result will be the same: big company has access to all your data.
The "Like" button probably wouldn't work though. If you're some external website, which "like" button will you embed? Google's like button? or Microsoft's?