Fincher should take solace in the fact that AMPAS is far from the end all, be all on greatness. Instead, they're just an organization that usually awards those films that they can relate to (i.e., those about privileged people whose biggest concerns are about shit like lisps or about not being able to find the perfect partner while using their wads of C-Notes as tissue to blow their stuffy noses).
To be fair, The Social Network is a film about wealthy, entrepreneurial Harvard kids suing the shit out of each other. So yeah, it was his best chance yet.
I'm not going to pretend I wouldn't be so happy to win an Oscar that I'd enthusiastically beat a tramp to death with my statuette, but I really don't assign much weight to the awards any more after so many hugely questionable picks over the years.