How to start a conversation with a girl you've only spoken to online?

[cici]

You go to the same college? And you both knew each other's identities even though you only talked online??

I'd make a joke out of it, like "hey, it's my olympics-tweeting-buddy!"
If you don't really have a way to approach her in real life at all in the first place....can you send her a message and say "Hey, I've always though you were an interesting person. Since we go to the same college, want to get a coffee with me and chat?"
Just be friendly and attempt to get to know her first off. She might think it's weird, or not want to actually have a relationship/know you in real life past hearing what you have to say about the Olympics (maybe she only wanted you for your tweets). She might not be in a place in her life where she's interested in meeting new people, or meeting guys. Give it a chance, but don't put too much pressure on the situation, because it seems kind of like a shot in the dark.