IIs this a good idea? What should I say?

Yea, I'd hit her up on facebook. Just be like, "Hey, I meant to talk to you about something at the graduation ceremony, but got caught up with the band. Would you happen to have time to talk sometime?" I mean you don't have to necessarily say those words, but just be casual and straight forward with it. It's best not to beat around the bush otherwise you'll never get what you wanted to accomplish done. Be confident.