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Write your number down on a piece of paper and get said mutual friend to give it to her.

She doesn't use her's anymore? hmmmmm How about texting? Ya got cel phones right?

Anyways why not chat with her on FB for awhile before you go up to her and introduce yourself in front of her friends. Get to know her that way, then hopefully you two can meet up alone in the...

How did you find out about her having a crush on you?