Anybody trying to compete with Facebook now is setting themselves up for failure. Even Google could not make a dent. We need to wait for fb to make a blunder. Right now, they are at the top of their game.
Google's social networks didn't really take off for a lot of reasons, mainly because they weren't really offering anything new that people would find preferable over what they were already using, which was FB or Twitter.
I believe that the more likely course of action, as others have stated, is facebook will make mistakes which will be capitalized upon. Large organizations gain cruft, slow down, choose the wrong direction. Many things can happen.
The more interesting competitors tend to pick a narrower use case (say, sending secret pictures) and become really good at it.