Am I annoying him or being too desperate?

I have a a crush on this guy. We routinely talk online (and in person) and we generally have good conversations, because we get along very well. In the past month or so though, I've noticed he sometimes won't respond to my instant messages. I'll say something like "hey" and then a while later he'll sign off without responding. The first couple of times I sent him "angry" messages jokingly that he didn't respond and he said sorry and gave some reason or another. But lately it's happening a lot, and since it's summer I don't see him as often and I miss talking to him. A few days ago I sent him three IMs (all about 20 or min in between) because I really wanted to talk to him but he never responded. So I sent him a facebook message asking him why he wasn't responding but even though I saw he was online, he didn't respond to that either. I know that I sound really desperate, and it's bad but I just don't know what to do. We're good friends, why wouldn't he just tell me if he didn't want to talk to me?