Ciel, you have an infatuation to this guy who doesn't know who you are. I'm not trying to say that to break your heart as if there's no chance in the world, but that he doesn't know that there is a girl like you, who think about talking to him constantly and continue to run into him but is too shy to initiate a conversation. You say "the girl who gave him the most random msg on fb" but he has no way of really knowing that yet, does he? You say in your OP that you used a anon account or something?
Shyness is a syndrome developed over time from comfort. It is a cocoon to be broken out of. It in no way defines part of who you are. You need to contact this guy. Somehow, talk to him. The best way would probably be to do what Thistle said, and just casually introduce yourself as a graduate from the same high school. It's not unusual for you to recognize him if he was a good member of the sports team, so he won't feel weirded out (for lack of a better term) if you mention that's how you got to recognize him around the school. Try not to mention the fact that you constantly run into him, because that might bring up the elephant-in-the-room: "why didn't you talk to me earlier?".
Just go for it.
If you feel like you could never in a million years talk to him just like that, send him a message, on a social site where his profile picture is clearly visible, and mention your connection to him from HS. Be friendly but brief, and end with a good question he won't have to think hard to answer, so he has the option of continuing the conversation.