> I guess all that growth must come from countries that recently got into Facebook.

I have worked for companies with big, really big, user bases. As time passes companies get way better at getting more value from each user. They have more data. They have better software to offer promotions, advertisements, etc. You may be better off with half the users but getting more money from each one of them than with more users but giving low value. Depending on where the lines in the graph cross less users and more money is possible.