The price might actually be adequate - they're not getting 200+ dollars in ads for the average user (IIRC it was something like 50+), however, these aren't average users; if someone is the type of person who's willing to pay 200+ dollars for Facebook, then showing ads to them can easily be worth 4x as much as the average user, that is the kind of disposable income for random conveniences that many advertisers want to get and are willing to pay a premium for.