The sensible answers here are from pwolf1703 and jpow1981. Basically, if she is refusing the number but offering her facebook, she isn't that attracted to you yet. She is comfortable with you and likes you on some level - if she didn't she would just say no without offering her facebook. Just stop talking about the phone number (just drop it, don't try cocky comments here. They won't work as no matter how witty you think they are, by saying them you are drawing further attention to the fact that she doesn't want to give you her number. You want her to forget this!! So drop the cocky comments here.). Go back to conversation and hit on her a little more. Then try again. If she continues not to give you her number, she's not attracted to you, move on.
That's the short advice, and that was all I was going to write, but then I saw this.....
Quote:
"Hey, why don't you give me your number?". If you phrase it like a negative question, in order for her to refuse, she will need to think of a reason NOT to give you her number, which puts the pressure on her to think of a reason why. Women don't like confrontation, so the easier option is to give you their number.
"Can I have your number?" is wrong because it DLV, you shouldn't ask for her permission to do anything, your needs always comes first.
I have to say Hunter_Foxe, I completely agreed with all of your first paragraph. But this is just wrong. "Can I have your number?" is perfectly fine. I got a playboy models number by just asking for it. You don't need to phrase it as a negative question, and more importantly -
while everyone doesn't like confrontation - saying no to giving out her number isn't really very big confrontation. If she doesn't want to give out her number, she won't, no matter how you phrase the question. Conversations between people don't break down when someone uses a double negative, and a negative question is not kryptonite to women

. This is just not in any way true. Sorry!