Stay off of Facebook. It shows too much interest that you have actually clicked on her profile and taken your time to write something.
When you have conversations at school, it just shows that sure she is a person you could small-talk with on the way to the next class, but when you move it to Facebook, you are trying too hard. You should just keep on smalltalking with her, and throw in a few light negs once and a while, and KEEP IT COOL
So to sum up: Keep the game at school for now, unless she shows countless IOI's to you.
PS: The thing you wrote on her facebook-wall, was it something that would lead to a response from her side? Or just something like "Hey nice talking to you today" (which sounds lame)
PPS: You CAN use facebook actually, but ONLY for some light negs. If you do, make sure it is clear that it isn't an insult but just teasing, cause sometimes that can be a bit hard to identify on text. You can use smileys for example.