If you still have class with her, I would keep most of the interaction there. In my experience, I have discovered that women lose respect for a guy that mostly talks to them online when he still sees them in person. This shows a lack of confidence, because online conversations feel less risky.
If you still feel that you need to start an online convo with her through facebook, making fun of her profile picture is always good if she can handle it. Or you could make fun of one of her favorite movies like, "Ah, I knew there was something a little off about you! You like
(random movie)? I'm sorry to say, but this will never work out. We're totally breaking up!

"
Let us know what you decide and how it goes. UPDATES!