It will have to decentralize, because it is not possible to run one community with one set of rules at this size. The contradictions are already intense and only going to get worse, even before I consider Facebook's most psychologically-likely reactions, which generally involve the short-term trap of gathering more power and exerting more control in an endless spiral. To quote a famous philosopher, "The more you tighten your grip, [Facebook], the more star systems will slip through your fingers."

The fact that this will kill Facebook is irrelevant. The fact that decentralization is observably very difficult is irrelevant. The hardness of a task does not affect its necessity. There is no solution for Facebook that leaves Facebook recognizably like it is now. It's arguably a fluke of politics and culture that got Facebook to where it is now.

(When I say "decentralize", I don't necessarily mean full on Diaspora-style decentralization. Decentralizing into multiple social networks that are still architecturally centralized is viable, though those social networks will face the same pressures at scale.)