If you want to answer this question, it's probably a mistake to look at Facebook itself. The biggest danger to successful technology companies is that the world will change in a way that makes them irrelevant. The new thing, whatever it is, will initially seem unimportant. By the time the incumbent realizes how important it is, it will be too late.

This sort of generic answer is not very exciting. It would be more exciting if you could say what the new thing would be. (To some people at least; to most it would seem a toy.) But that is quite hard to do.