Kickin myself now...?

Last weekend my friends and I went to a local bar (age - early/mid 20's). I saw a girl that I thought was very attractive and wanted to make a move but ended up not making a move. She left fairly early but had more than enough time to make a move. She kept looking over at our group and one of my guy acquaintances said he though she may be interested and I should make a move to talk to her. Regrettably, I didn't do so. Also, my one friend said he recognized her from high school although they were not friends, nor do they know each other well enough to speak at a bar. I was the only guy in our group that wasn't their "together" with someone. Perhaps, the girl to whom I was attracted was simply trying to figure out if she recognized my friend from from their high school days. Since my friend knew the girls name I ended up finding her on Facebook. How weird would it be to randomly message this girl on Facebook? And if you think that is a good idea, what would be the best way to phrase it?

Of course the ideal situation would be seeing this girl around again and I know I'd make some sort of move since I'm still kicking myself a week after the fact. However, my friends and I go to this bar fairly regularly and this is the first time I've seen her there. I don't exactly live in a small town so bumping into her again would just have to be pure luck.