I've watched the lifecycles of a LOT of social sites, from the beginning days of BBSs. Lots of "but this one is different" sites. They have all gone thru pretty much the same lifecycle ending in stagnation & abandonment around 7 years after starting. Some of them (CompuServe, AOL, MySpace, others which were just as big and even more forgotten) took the social tech world by storm, were poised to take over the world, and garnered lots of investment because they were so darned unstoppable. Most users got bored and moved on. Like Zaphod Beeblebrox, they're dead - they just haven't stopped moving yet. ALL OF THEM.
And Facebook shows absolutely no meaningful differentiation from that pattern.
(I want to say "now get off my lawn", but many may miss the intended humor.)