I guess I am the opposition as well. My almost 16 yr old has had MySpace and Facebook. FB is by far the best one. I am on it a lot as well.
She is all honors, co cpatian of cheer squad, orchestra, and it is not harming her. If anything, a prospective employer would see the positives.
Of course some of the kids have horrid language, bully and yes, at some point an employer would not look kindly on it, HOWEVER....they are kids and not expected to act like responsible adults at 13. GEES.
Set the privacy on the highest levels, be open and communicate with her, I have daughter's log in and password and I check at random times.
If you are smart about it, you can have it and it work out well for you. She can't live under a rock all her life you know.
Be open, honest and communicate with her... If you start forbidding things, rest assured you will be left out of the loop and she will go behind your back.