If the charge was $1.00 (the charge formatting was a little unclear), this is likely a typical verification charge used to verify that your credit card information is correct. Most services void these charges as soon as they get a success/decline message from payment gateway processing the transaction, and you should see this charge removed from your statement in about 24 hours.
This charge is vaguely referred to in their payment TOS: http://www.facebook.com/payments_terms/
"If you pay by credit or debit card we may obtain a pre-approval from the issuer of the card for an amount up to the amount of the purchase...", although not clear that this also happens when there is no actual purchase, when the card is just being used as a verification method. Definitely an opportunity for Facebook to clarify the behavior when adding credit card information for different reasons.
