About this girl I messaged on facebook?

[Lawruh]

I do the same with a lot of people. I'm just not in a chatty mood at the moment, but It's not like I don't like those people. Maybe your girl does the same.
I say, comment funny or interesting things on her status, or places where she could clearly see them so she could see what you're sort of like. Or if she does any of those "Like my status for a whatever on your wall" thing, like it. It'll provoke her to talk to you when you message her.

Also, maybe she's the type of girl that doesn't like simple and plain talk like "Hi, what's up?"
She could be the kind that likes lots of playful humour. Start off a coversation with a funny pick-up line. (My favourite: Do you have 11 protons, because you are sodium fine!") but make sure she knows you're only joking. It might creep her out if you're serious.
If you really like her, keep trying but don't act crazy stalker-ish. Good luck.