I think 12 is a bit young for FB... to be honest. We ONLY allowed our 13yr old to have one cause that was the age limit, and truly, we didn't like it-- but her mom (who she only see's every other weekend) wanted to set one up for her, and really, even if we said NO, she would have done it anyway... so we just agreed and we have the login/password so we can regularly check her page and delete anything we don't like. But if she was being harassed on it- I would delete it all together. Kids get enough of that crap in school... and don't need to be exposed to it online. I would, however, make sure to copy the things the girls are saying or keep track and know WHO they are so you can go to the principal at school if needed. Harassment, even online, at my kids school is NOT tolerated. And parents are advised to keep a close eye on what their kids are doing while on FB or other social sites.
The other option is to delete those particular girls and block them from even seeing your daughter online... although, if they have friends of friends, sometimes they will find a way to find her regardless. It's best to protect your daughter however you deem necessary...
I'm sorry she is going through this. My kid has some mean girls at school who like to call her names- cause she is cute and outgoing and has lots of guy friends and has no issue talking to boys... and other girls think that means she is this or that... it's SO frustrating as a parent to hear about.