How to ask my BFF for her number over facebook?

You don't want to be too over-emotional if she's your best friend, because if you do she might find it hard to talk to you about it! Just be how you'd usually be with her. Say something like "i'm going to miss you, do you mind if I get your number so I can text you to meet up at some point in the summer?" And then that leaves it open, so she can take it either way! Sorry if this didn't help.