>I barely trust Paypal, you think I'm gonna trust FB with my CC details. You gotta be kidding.
Trust isn't as big of a deal as you might think. People think it matters a lot in business because it matters in your day-to-day life with in-person interactions. The fact of the matter, however, is that many people who don't trust things still use them.
I don't trust my government, but I still pay my taxes and use the clean water they pipe to my house.
I don't trust most banks, but I still use their CC processing networks.
Plenty of people don't trust BP (and oil companies in general) but still drive internal-combustion cars.
More and more people every day don't trust Foxconn, but still buy all the gadgets they produce.
Trust matters when you are dealing with something non-ubiquitous; you don't need to use it, so if you don't trust them you don't use it. However, if something goes main-stream (like credit-cards), and the interaction is practically forced on you (like taxes) then people use it without trust. Facebook as it stands right now is testament to that. I don't trust them _at all_, but am forced to use it because friends, family, and groups I personally value use it in a way that requires me to as well.
If they launch a payment processing system that even only a quarter of their users are willing to use, that is enough market share for websites to start using it, which is free advertisement for the system, gaining more users. How long before sites that _only_ take facebook pay crop up (in the same way that some sites _only_ use paypal; a company plenty of people don't trust, but are forced to use)? How long before some site that provides a service that you depend on?