This guy I like talked to me a lot for a few days, then started to barely speak to me?

This guy goes to my school, but I first started talking to him on Facebook. I basically messaged him, joking around about how he looked like a character from a show I watched, and we started talking. Usually we'd each send one message per day, sort of in a continuous conversation. Then one day, we finally had an actual, consistent conversation. It seemed like we were getting along well, and we talked for hours and hours. We did the same thing the next two nights. After that, though, it went back to him responding once every day. So, I figured it was because I hadn't been online from my laptop, just my phone, so it would show me as offline to him when I wasn't. I sent him "Oh, just letting you know, even if Facebook says "offline", I'm probably not, and I'll answer you whenever you happen to message me." The next day, he messaged back saying "Not going to be online that much. Too busy. I'm not a person who uses Facebook alot..". And continued answering me once every day or so. But lately, he's been answering me maybe once a week. Whenever I happen to see he's online and message him, he talks to me for maybe 5 minutes before saying he'll be back in a minute and never coming back.

In school, he kind of ignores me, but it doesn't seem to be purposely. I sit with him and a group of people who are friends with both of us at lunch most days, except for when he's not at school or has work to make up during lunch. He's a bit socially awkward and quiet at times, so I try not to take it personally. I've asked him if he was alright before while sitting with him alone at lunch (our friends had gotten themselves lunch detentions for not doing homework) and he said "Yeah, I'm just usually really quiet.". Around his friends, though, he's not quite as quiet. He's still kind of quiet, but not as much so as he is when he's talking to me.

Why is he acting like this?