Well just go with the flow I'd say. For now forget that you ever asked for her number. Just show some genuine interest (not as a buddy though) in her life and share stuff from your life, in short create some kind of a connection/rapport and then flip out a "So.. do you it's time to try to get to know each other even better?" or something like that. This is where indirect approach works well because the direct approach has already been registered once.

Also, if she isn't online too often then to be very honest facebook isn't the best way to approach because it won't leave a lasting impression of yours on her mind. Find some place which she visits often or some place where you can get to talk to her. You could ask her out for a casual coffee where the two of you can get to know each other well.