How should I approach my daughter about her Facebook account?

[Lewis]

You should have-Talked her through the basics like dont add strangers......and so on.
Then if she understands, you set up the account making sure only friends and family can see details.

But going behind your back and getting her one anyway was bad, talk to your wife and tell her your daughter will never listen to you if she keeps going behind your back and letting her have what she wants anyway.
Facebook is not a bad site if you play your cards right....and if you ever accept it....tell your daughter she can tell you anything and not to keep secrets from you.

Whether you decide to delete her account or not....make sure your friends with her and check her page everyday.