Tread with caution. I've found that women who hand out their number so quickly turn out to be really full on and/or fake. Personally I like to screen the people I talk to for a while.
If she makes an excuse about Facebook take it as a minor red flag. A lot of these chicks are on FB, but their POF photos aren't current, while her FB ones are much less flattering, or she could be in a relationship.
Of course, she could be telling the truth, but in that case you have to wonder about the drama she mentioned.
Personally, I'd spend a bit more time chatting to her on POF before giving out your number. But if you want to take a chance I'd say a simple "Hi, it's your POF friend Adam here. What you upto? x"
A simple introduction will get the ball rolling. Then you can go from there to asking what she does for work, hobbies, what she looks for in a man, if she's dated online before and then escalate to some flirtation and asking her out.