I think the big problem, like others have said, is that you didn't have an agreement about this beforehand.
I personally am pretty conservative about computer stuff. When I had preteens and teens, facebook wasn't around, but myspace was. We kept the computer in the common area of the house and we were *very* cautious about myspace. I know facebook is supposed to be "safer" than myspace, but I am on facebook and I just don't think it is appropriate for pre-teens and young teens unless the parent and kid have set it up jointly to be a shared thing (no secret passwords, etc.). My brother-in-law who is so protective of his girls in soooo many ways let them have facebook accounts, and they have "friended" one of my other neices (their cousin), who is older and posts stuff all the time that is so super inappropriate. If they were talking in person, I know this older cousin would be more careful about what she says around the younger girls. But she doesn't think about them when she posts crap on her facebook. She just posts. I think for that reason Facebook is a problematic "setting" for younger teens.
That said, even though my current kidos are younger, I'd probably allow an account some years down the line (right, like facebook will exist then!) and do it as a shared thing because I wouldn't want my kid to sneak and do it behind my back out of curiosity/desire to fit in/etc.
When they mature into more "solid" teen years, I'd back off and give them some privacy. And personally, I see a lot of parents who are having trouble letting go in the teen years these days...and the kids have zero privacy which is problematic too. So I do see the flip side.
Anyway, OP, I'd feel badly if I did what you did too because it really violates trust. But I wouldn't feel badly about setting up boundaries around facebook that include a shared experience with it...12 years old is still so young!