I don't want to scare you, but you can get in jail becaus it's fraud it was recent on the news. But if you just say to the person that it was a joke and remove the facebook , than there won't be any problems ..
Reported. Hope Jagex comes to your doorstep Just kidding, you should be fine, it depends on if the girl is you know... Crazy and would do something like get a lawyer and sue you for impersonation. Is that possible in America? I'm not sure. If you are from America, that is.