Facebook--Pandora's Box?

After investigation, we discovered that at thirteen you can have a Facebook account.  I was approached about this since my daughter has been thirteen for "almost two weeks now."  Our daughters each have an email account.  They know that at any time my husband or I may log on and read anything and everything that comes in or goes out.  They think we have spyware that can retrieve deleted emails, so they better not delete anything.  (We may have this---I don't have any idea about spyware).  We periodically review the technology agreement they sign for school (private, religious school) and discuss scenarios of do's and do not's.  But Facebook is a whole other issue.

I really struggle with this.  I flat out think she's too young; however . . .I see this as her social outlet.  As kids, we'd walk to and from school with our friends, go to the ice cream place, talk on the phone, etc.  This weekend I put 40 miles on the car just driving to and from friends' houses for sleepovers.  They kids are bussed to school.  Again with Facebook, we reviewed a bunch of things--especially that EVERYTHING you put on-line could be there PERMANENTLY.  I am the only adult allowed to be on her account.  She may not post pictures on her account w/o my approval.  I have made her password the same as her email password.  She knows I will be checking.  She knows that I will delete inappropriate entries and defriend inappropriate friends.  Even after these discussions, I know something will pop up.  (Like that I know she logged on at midnight at a sleepover this weekend and was having a conversation with a friend).  Have I just opened a Pandora's box?  Or would it be worse to keep that box closed?