IMHO, the rising of Facebook and Twitter is due to email spams defeated email user experience in 2000s. FB and Twitter provide a much higher signal to noise level than email for common users without knowing spam filters and all other craps.
So maybe the thing that replaces FB or Twitter is something that makes my communication with my family and friends better. What will that be, we will know when it rises.
Or, the consequences of privacy violations will defeat the social networking user experience in 2010s, and others are going to provide a platform that enables better communication with family and friends without reverting your privacy settings every other day and retroactively opening up all your data to everyone ;-).
So maybe the thing that replaces FB or Twitter is something that makes my communication with my family and friends better. What will that be, we will know when it rises.