Easy mate been there before. A lot of ppl say its creepy, but not if you do it the right way. Here is how I do it and I've never had a problem.
1) Add her on fb.
2) Send her a msg something like this
Hey your page came up in the ppl you may know section I thought hey I know this girl (from whatever your training was) lol. Its too bad we didn't get to talk a bit longer you seem like a cool girl. Anyway I have to run enjoy the rest of your weekend
3) Wait for a reply and respond appropriately.
In case you're wondering I have done this before sent that exact msg. And I ended up dating the girl for 3 months. If its you're only crack at her go for it you'll regret it if you don't. I'd personally suggest you look up Gambler's material on FB game it may help you out a bit.
Anyway hope that helps good luck!!