How to start a conversation with a girl through facebook (I haven't talked to her in real life)?

Ok, so, there's this girl I have a little bit of a crush on, and I'm pretty sure she has a thing for me too. When I was still in school (I'm a senior so we get out sooner), we often exchanged eye contact, like she'd turn around or to her side and glance at me when she walked to her class. One time, her and her friend kept turning around and glancing at me. Another time, I saw her with some other from the corner of my eye, so I turned to my side and our eyes met for a few seconds. Then when I turned away, I heard giggling coming from their direction. She's smiled a few times, but at the time, I wasn't completely sure it was intended for me, so I didn't smile back, I just kept my eyes locked on hers. I promised myself I'd approach her, but whenever I'd see her, my heart would start racing then I'd get super nervous and not do it. On my last day of school, when I was waiting in line to get my cap and gown, I saw her and her friend. I finally mustered up the courage to smile at her, but she didn't see me (I was in a huge crowd of like 300 people), so she just went to class. She's the only girl I've ever been this sure likes me back. Just the way we made eye contact, it felt so different from how it'd feel with other girls. And there's just a gut feeling. I ended up finding her on facebook tagged in one of my friend's pictures, and decided to add her. She accepted it, and now, I don't know what to do. I want to start a convo with her through facebook, but I don't really know how. Thing is, I don't know how to start a convo with ANYONE on facebook, boy or girl, if I haven't talked to them in real life, but since I don't have school anymore, I guess this is the only way of getting contact of her. So, how would I start a conversation with her?