Paul Ford of NY Magazine wrote in May 2012:
Peak Facebook, when it does arrive, is something that Facebook-haters should fear, not welcome. Facebook’s platform has been so overwhelmingly successful that the company hardly had time to do anything but grow. Yet when the growth of the network itself slows, as it too inevitably will, Facebook—as a publicly traded leviathan whose mandate is to increase profits—will need to find new ways of slicing and dicing humanity into groups that will respond to marketing. That’s what lurks on the other side of peak Facebook, and it is going to suck.
(I couldn't help but find a random peak oil graph and stick it up there...)