first i would talk to your school. most schools around me have their security and internet set in order that facebook is completely blocked, not allowing it to be accessed at all. also mention to the teacher or whomever that non school related internet access is going on and they need to do something about it to protect your kid and others.
does your child have an email address? you have to have an email address to get facebook, so im guessing he does. this can also cause a problem, so because of his disobedience, i would delete that as well.
you are MORE than right to feel like taking facebook away. require that he let you into his account and you delete it. (if you dont know how to do this send me a note and ill go through it with you)
calmly and firmly tell him that what he did was directly against what you had told him, and i would even go as far as to remove all internet privileges for a period of time determined by you (and your spouse/partner if applicable). tell him that when he is 13, you and he will set up an account together, and he WILL be your friend on facebook, you WILL have access to his email and facebook accounts (which means you WILL have his passwords!!) and that YOU will determine if he is doing something that is against your family's rules for behavior.
facebook is a very very popular "tool" for communication, but it can easily and quickly become something dangerous and/or damaging.
then i would try to figure out where this is coming from. typically kids dont deliberatly do something to rebel against their parents unless something is going on. only you will be able to figure out what that is. if your son is only interested in playing the games on facebook, tell him that it is perfectly acceptable for him to play the games on your account. if he wants to talk to friends, give him a time slot every night where he can call them on the phone without interruption or something. find out the REASON why he thinks he has to have a facebook that bad. ask him why he did it without your permission (although im not guaranteeing an answer, kids dont think things through they just impulsivly act dont they. LOL.)
anyway. good luck. remember to put your foot down, and follow through. whatever punishment you give him for disobeying you, follow through on it to the letter. be rational and calm, because that will get you farther than being explosive. im sure he might get explosive, but it is your job to be the parent and be confident in your parenting decisions.
but like i said at first, you MUST talk to the school. i cannot believe that there are still schools out there that have internet which allows access to those kinds of websites. ours are so blocked up that its literally research only. :P but thats the way it SHOULD be. either the school, or the faculty member supervising, need to be VERY SURE that computers are only being used for school related activities.