Well, try this on for size: You aren't a customer of Facebook because you aren't paying Facebook... Facebook is paying you. For the privilege of being able to sell your eyeballs, they pay you a great deal more value in their services. They're putting down a non-trivial amount of cash on providing these services to you that you put no cash down on.

Cutting you off at the scale they are at is just no big deal to them, but you may in fact miss your payment.