Stuff like this is never going to work, because it suffers from the chicken-and-egg problem. Everyone (who cares about Twitter or something like it) uses Twitter because everyone else uses Twitter. You can make an account on Quitter or Diaspora or any other decentralized FLOSS network, but it'll be useless because all your non-nerd friends (and most of your nerd friends too) won't have accounts there, and will contin…
I would have felt the same way about MySpace. The reality is, alternatives can arise. Facebook won't last forever, and Twitter even less so (although one of my favorite attempted "Myspace killers" was a social network for karaoke singers)
When has an open, decentralized product or service ever supplanted a centralized, closed, proprietary one? Never, that I can think of. Linux never replaced Windows, not for the vast majority of users. It did become very popular for servers, but really it took over from proprietary UNIX there, and really created its own market because back in the early 90s there was no such thing as a webserver; the whole use-case is different with Linux on servers. So UNIX was never really entrenched in exactly the things that Linux servers are used for now. Then there's smartphones and embedded devices like that, but there again Linux really created its own market; before Linux, small devices generally didn't have enough power to run a heavyweight OS like Linux. Android (semi-open, uses Linux) came about pretty much simultaneously with the iPhone. So here again, there wasn't some single proprietary thing that was dominant and entrenched in a mature market. There's the Apache webserver, but here again, there was no big dominant proprietary player. LibreOffice hasn't had much success in supplanting MS Office. MySQL is very popular, but here again there was no dominant proprietary web database; MySQL created that market (though PostgreSQL has become a serious competitor to it, but it's also open).
So in summary, the only time I've seen open, non-proprietary solutions win out is when the environment changed, frequently because of their presence and availability, making them the natural pick. I've never seen them beat out an entrenched dominant player in a mature market. So I definitely don't expect to see it happen with Facebook. What I could see happening is something coming up which uses a non-proprietary or decentralized system which creates a whole new market, rendering one of those things obsolete. I have no idea what that could be though, but then again 20 years ago I wouldn't have imagined that people would want to spend massive amounts of time on a "social network" sharing inane pictures and videos and making idiotic comments to each other about them.