Douglas Rushkoff posted an interesting article a couple of days ago predicting that Facebok's days are numbered. It's an interesting article and worth reading.
While it's inevitable that Facebook will fade eventually, I don't see that happening in the foreseeable future.
MySpace, AOL, et all declined because (1) they failed to continue to innovate as an earlier commenter pointed out, and (2) there was 'something better' for users to migrate to.
I don't see the 'something better' on the event horizon. And my sense is that the innovation continues. My opinion? FB has plans for all those 'Like' buttons we're putting on everything and those plans mean that if anyone should be worried about a decline it's Google.