The only correction is that in this phrase "Facebook works on the theory that when you have a lot of people, you don't have to make as much per person, because the amount of money you make is the number of customers times the amount of money you make off of each one" you confuse customers and users. Facebook users are not their customers. The customers are advertisers, and there ain't 500m of them. So, although their profit hugely depends on the number of users, it's not the only component in the formula.