Here we have a company with a very very large cross section of just about every culture and country on earth. It has massive amounts of personal. intimate. detailed data on its users, their relationships, their interests and activities.
We have a CEO who is young, idealistic, ego-centric and in certain ways, impressionable. Not in a petty sense - but in a way where providing the opportunity to feel as though he really is changing the world, he will assume that he is -- and for the better. The problem is that, I believe, he (and facebook) will be manipulated.
Regardless of Mark's ideals and lofty goals - politics is politics. It is a system of control in its basest form. Facebook is the perfect venn between social utility and totalitarian encroachment.
Sadly - the social utility for its users currently outweighs its encroachment and it will be ratcheted further into a position where we cannot separate our daily activities from it.