Ex keeps messaging me every couple of days, why?

My ex boyfriend and I drunkenly hooked up one of the last days of the semester before we both went home for the summer, during which at the time he said it didn't change anything. Now every few days he's been talking to me on facebook and is making random conversation that's all just small talk and nothing serious. We broke up a couple months ago and then until that night we hadn't talked for about a month. I'm pretty much over him, so I'm fine with the talking, I just don't understand making small talk when he's the one who broke up with me.