its probably because you need to verify a billing source, just go to settings, scroll down to app and iTunes stores, open it and press open where it hows your apple id, then click view apple id,type in your password, then open up payment information, and then put in a debit card if you have one or a credit card then finish putting in the card info. then download Facebook, go back to settings open up App Store and iTunes again and click payment information then when it shows your billing info for payment type simply press NONE then it shouldn't ever ask you again. UNLESS you have a paid app or song and the payment didn't go through properly... if this is the case call apple and they will give you a refund in 2 days the money will come its very easy.