The thing about Facebook though, is A) The interface requires lots of care and feeding to stay clean and appealing - Facebook has stayed nice through it small number of very talented engineers. B) Facebook's economics and its supply of topnotch talent has been leverage from its constant growth and its quality as the thing that's happening.
If it instead is faced with a slow decrease in users and a feeling that it's "jumped the shark(tm)", it will have a lot of trouble keeping its economics, its talent and thus the quality of the interface. So I imagine that FB could fall apart in some medium-scale time frame. Oddly enough, I think the end of Facebook wouldn't be good news for those plots "alternatives" to it.
I actually hope Facebook IPOs soon, might postpone some problems...