Since all game developers that integrate their products with Facebook now use the Credits system, your purchases will show up on your bank balance or card statement as being authorized by Facebook, regardless of the application you're making the purchase in. The first time you use Facebook Credits, you'll see a temporary payment for $1.01, which is a simple transaction used to verify your payment method. Once your financial information is validated as active, the charge is reversed. Keep in mind that server traffic may delay the immediate availability of in-game merchandise and/or payments showing up in your Credits account. If your account doesn't update four hours or more after your purchase, contact Facebook and -- if this applies -- the developer in question. They'll work to remedy the situation.
For those worried about transaction security, Facebook provides on-site information and customer service to help you understand how the payment system works. Remember, it's in Facebook's best interest to keep its users satisfied, which means keeping an up-to-date FAQ and Help section as well as responding to your needs in a timely way. On the other hand, as a consumer, it's up to you to monitor your account balance and usage, as you would your credit card or cell phone usage stats, to make sure nothing crazy is going on.
The bottom line is that Credits are an easy, smart way for Facebook to keep your experience with the site going -- the less time you spend surfing to other sites to make payments, the more likely it is that you'll keep playing that game or using that application, keeping your eyeballs firmly where they want them. In the end, it's your choice whether the fun you're having is worth the Credit -- and your responsibility to remember that, even at just 10 cents a pop, those things add up. As anybody who's gone through app addiction on their new phone -- which is probably all of us! -- can tell you, that's a lesson you really don't need to learn more than once.