My money is either in 1 of the 3 giants (GOOG, APP, MS) finally getting the social web right or Twitter.
But the problem with Twitter is it has fumbled countless times already and is only growing due to the strength and type of its community. So i have no confidence they can catch up to Facebook.
So Im guessing if its not the 3 giants, it will be an inter-operable social layer and a thousand private social networks.