http://en.wikipedia.org/wiki/List_of_social_networking_websi...
Facebook has the most users in one place, but the dust is nowhere near settled on where everyone will end up.
This bodes very well for a decentralized solution. I doubt Diaspora will be the first (I think Appleseed (http://opensource.appleseedproject.org) is much more likely), but ultimately, it will be an easily implemented protocol with mature cross-platform libraries that'll really take it down.
This is a good PR move on Facebook's part, because it willfully misinterprets people's frustrations with Facebook. It's not that you didn't have a local copy of your data, it's that you had no control over your data when it was out in the wild. This will take some wind out of the sails of the decentralized solutions, but the armada is still coming.