How to get a girls attention?

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I sent her a message on Facebook yesterday saying..."Hey! I'm not sure if you remember me from _____. We were in the same art class sophomore year, I believe. How've you been?" This was my first message by the way and my first time I've talked to her on Facebook. She was active online twice today, but still didn't reply...some girls already answered this and said, she might be busy or creeped out. What should I do? Back in high school she told someone I was cute, but I never had confidence to talk to her back then. Should I comment on one of her pictures to get her attention and then she might realize I sent her a message?

She's not in a relationship.