How to ask this girl out?

[Taurus the bull]

Let's keep this short and sweet...
I met this nice, sweet, and beautiful girl in a class last quarter. It was obvious she liked me and I definitely liked her with lots of evidence from class and our study group. I didn't ask her out then because there was a "complicated" issue with a previous boyfriend (not sure if true or not because some girls do lie to spark jealously) because he went to a different college out of state. She seemed confused from her responses regarding him and I didn't want to put a burden on her, get involved, or cause future issues (love triangle). Now the quarter has ended, we're in winter break, and I realized that I probably won't see her again after this (same major but different class levels because shes a freshman and I'm a sophomore). ...and I'm sort of freaking out!

Now I'm stuck because I don't have her number and my only tool is Facebook. What do I do? How do I ask her out or get her number to ask her out?