I'm leaning towards "no". For any site to replace Facebook it would require either many, many users to make the transition to another site or to outright leave making the site less useful for its users, and I honestly don't see that happening. Facebook is honestly nicely designed, has plenty of users and I can't see any killer app or nicer equivalent of Facebook appearing to make moving worthwhile.
The only thing that could replace Facebook is an unprecedented sharing of data between many significant players allowing interoperability between sites much like how E-Mail works. The existence of DataPortability may be a slow but sure approach towards this goal but there's too many factors to consider influencing whether this will ever actually happen. I certainly hope so in the long run - one site having the amount of influence that Facebook potentially can worries me.
Disclaimer: I don't actually use Facebook, and probably never will.