How is this different than any other random girl you’d approach? Just because you became facebook friends with her doesn’t mean you should now have a different approach. I would do exactly the same thing whether or not you are facebook friends with her. Facebook means nothing. In fact, I avoid being facebook friends with lovers.
Approach her direct. Approach her indirect. Approach her any way you want. Don’t make reference to the fact that you’re facebook friends. In fact, if she brings it up, say, “What?! We’re facebook friends?!”
Eric