I really don't want that to happen. Sure, it may be a good idea for them, but I'm already tired of services that won't let me register without a Facebook account. When it comes to payment, they better have a good alternative available.

With credit cards we have a number of "hidden alternatives". We've got multiple processors, multiple card companies, etc. It's a number of layers there. With Facebook and Paypal... you're not able to pay when they say you're not able to pay (or be paid). They're not even controlled banks, so there's little you can do about some issues.

Think about all the problems when Google cut someone off and didn't provide any way of contacting them. Do you have a good way to contact Paypal, or Facebook now? (besides forms that result in canned responses) What are the incentives for changing it?