For a long time Facebook wasn't available over seas. Facebook started with only US colleges, then high schools. It only opened up to people outside the US about 3 years ago I think so I would assume a large percentage of its user base is american.
This is as insightful a theory as saying Facebook launched 7 years ago and now it has 850 million users, so on its first year at Harvard it must have gained 121.4 million users.
