I don't have a credit card. I know a lot of people who does but rarely uses them, debit cards feel more secure than credit cards against theft/fraud (and I'm not really sure if they are, I don't think so).
Visa is a credit card, however. In Europe, it's often tacked onto a regular debit card, but it's a separate thing not everyone has.
In europe certainly most people would seem to have a debit card. Besides I'm assuming at the backend the FB system would require you to enter a debit/credit card.
It would seem very unwise for FB to begin taking down everyone's bank account number as in the case of fraud with a card you can simply cancel the card and get a new number.
In Brazil, I do not know a single debit card that will allow international purchases, or that will be allowed in Paypal (even though Paypal has a local presence).