I know this is lame, but what's a good excuse to talk to a guy on facebook?

We used to go to a tennis group together, but never really talked. I'm very shy and it's hard for me to tell someone I like them or give them any hints that I'm interested. To add to that, I'm always tired of being single :/ ..Anyways, last month I went to a prom with one of my friends and he was also in the group that went with us. Like I said, I'm terribly shy, but I noticed him glancing at me while we were eating dinner. I muttered a few sentences at the table, felt very uncomfortable, and figured I had ruined everything with the people I was going with. I got home and he had added me on facebook and sent me a chat, but I accidently x'ed out of it instead of opening it. He got offline before I could say anything back.. A month later, I'm still thinking about him and I really want to talk to him. I feel it's too late to ask him what he messaged me about, but I really want an excuse to get to know him. What would be a good excuse?