Long story short;

6 months ago i started in high school, back then at a party a girl told me that she thought i was pretty cute, but I had a girlfriend back then (not published on etc. facebook, very few knew I had a gf). I did feel the vibe, and if I had been single I would've chased. Now, a month ago, I broke up with my gf, and I ended it immediately on Facebook. Now a week ago, at a café, (My high school has café every friday, where you can come chill, drink a beer or two.), she started looking over at me. one time, two times, 10 times, 20 times, I didn't count, but it was pretty obvious that she was looking, and our eyes met a time or two. I think I did a good impression by smiling, but I was busy, and therefore I didn't walk over to her. Now here is the real question; Facebook, what to do? would it be a good move to like, say, like her profile picture? I haven't talked to her for quite some time, actually the last time we spoke was like 3 months ago, at a party.

EDIT: TL;DR: Is Facebook a good tool for letting a girl know that you are interested?

EDIT2: The reason for why I am asking, is because I rarely see her in the hallways. I have finally convinced myself to go talk to her, in a sober condition, but it is just so rare that I see her, and therefore I thought to myself that it could be a good way to let her know, that I am interested and therefore, perhaps building some momentum for the next party on our school.