Why is it charging me to download facebook?

You will NOT be charged to download ANY free app. But you MUST have a valid payment method on your account to download free apps. The reason is almost ALL free apps, Facebook included, offer In-app purchases. And that means in order to download the app, you have to have a valid payment method. Again, you won't be charged for anything, unless you want to make an In-app purchase. If you don't want to have a valid payment method on your account, you can't have apps like Facebook on your iPhone.