got facebook instead of number

okay some background info! i work at a day camp, and there is this girl, HB 9 from another camp (same company, different location) so we have been going on field trips together once a week since summer started and we always gave each other looks. so since today was the last trip of the summer it would be the last time i was going to see this girl. so when we divided all our campers into groups i decided to go to to the group she was in so i can make my move. keeping in mind im at work with children everywhere i did what i could in terms of Comfort building and rapport. had a good conservative conversation and such, i kinoed when i could, without my boss getting suspicious lol. so just before lunch starts im walking with her and i say something along the lines of "hey ill do this now while i still have the chance id love to get to get to know you better outside the confines of these day camp walls, give me your number and we will have a good time". so she says i can have her facebook instead, i tried once more to convince for the number but failed so i figured i might as well cut my losses. rest of the day was fine, smiled at eachother and talked when we could (we barely had a chance to talk after lunch)

now! my question to you guys is how to approach this, im not a big fan of facebook game so im not very informed or knowledgeable. i would rather message her first rather than add her off the bat. a line ive used before but not as an opner (when she asked why i havent added her yet) was "i dont add strangers but if it makes you feel better ill add you after our first date". that worked really well so i wanna pull it off again.

anyways if anyone can help me out or direct me on how to start it would fantastic, shes actually gorgeous! so challenge accepted lol. thanks a lot guys.