How should I get with this girl?

ok so i went on spring break with some friends and their families, including this girl that i went to high school with. We knew each other, but we had never talked before this vacation ---- I am a freshmen in college and she is a junior in high school now btw, but were only about 1 actual year apart in age.anyway i spent most of the week of the vacation just getting to know her and toward the end we started to "click" but i had to fly home by that time. we both expressed how we should hang out this summer.the problem is that i didn't ask for her number, but i did friend her on Facebook. i wont see her again until i go home in may for summer, and i don't know how i should go about getting together with her... should i get the number by asking her through facebook--to me this is somewhat cheesy? or some other way? also im hesitant about getting it now and not be able to call her until summer when im actually around. should i wait?