FB allows you to buy gift cards from stores for credits, so for an outlier that doesn't trust Facebook with cc details, there's another option. Through buying Karma, they can bring gift giving into Facebook credits, so for someone's birthday you can buy a gift card through Facebook, and you can take your phone to the store and use your gift. This has tons of potential and you can for even more through Facebook. Think of buying tickets for events, or going to a small business' store and buying an item (Etsy style). Or Etsy, Amazon, eBay, etc setting up apps where you can use Facebook credits to buy (long term). I have two cousins visiting from India, and it's amazing watching them use Facebook. They're constantly on. Using it to connect with friends, play games, etc. This is a whole market the most people don't think of. These markets are huge. And as another user pointed out, all these kids using Facebook these days are growing up with it and will continue to use it, and trust it. It's a whole different experience that we don't really see.
