If you want to get a girl's attention you should compliment her. It shows that you don't care if other people see it. You shoudl be confident enough in your feelings for her. It would flatter her. I knew this guy and he told me that he liked my profile picture on my wall, and it was enough to have me smiling for the rest of the day.
What to write on her facebook wall to get her attention? Hmm?
If you really like her you can man up and handle it in person, I realize that people facebook more than talk these days, but that's not an excuse, If you really like her, you'll talk to HER about it, not post it on her wall.