It really depends on the situation in which you met her and the time frame.
Assuming that you had enough interaction with her to get her Facebook I would do the following:
Comment on something she posted recently, but argue against it or say something funny.
Example:
Her: Omg, school is so hard and exams are gay blah blah blah.
You: Love exams. Hours of studying and 20 cups of coffee per day. Whats not to like? (sarcasm)
And then....gauge her response...if shes positive and responds. Shoot her a private message:
You: Hey! Whats your digits? I'm going to add it to my rolodex of semi-cool people.
Through the above process, I've always received a number.