Why do guys do this? I just dont get it?

Omgsh! I don't know if this happens to anyone else,but when I'm on aim and a guy ims me, I start talking to them and they dont say much other then hey whats up, and then maybe a few other things. I've had 5 guys do this to me, like they are always the ones who im me and whenever i try talking to them or ask them something they answer but dont keep the convo going. Idk what it is! whether they are just iming me jsut 2 do it and annoy me, or if me responding freaks them out(doubtful cuz they wouldnt im again)