Why does my ex-boyfriend occasionally message me on Facebook?

[Alex]

We were never very serious, only lasted a few months, and we broke up due to lack of communication. It seemed as though he had already decided he was going to stop seeing me, but strung me along for a little while and then finally vanished. He wouldn't even have a conversation on in person, on the phone, via text or Facebook explaining why he was breaking things off. I know, kind of shady.

After that we went a month with no contact. Then, one drunken night I Facebook messaged him asking "what happened?" and he got back to me explaining why he stopped seeing me. It was kind of a small and silly reason, but at least he finally told me.

Two weeks later was New Years, and he messaged me saying Happy New Year and asked how I had been. That was all.

Two weeks later it happened again. This time I was in Vegas when he asked how I was and what I was up to, so I told him. The conversation ended after a couple of brief messages.

Two weeks later (TODAY) he just messaged me asking how I am and how the rest of my trip was. I have yet to open the message and reply and how should I handle it? I still have feelings for him but I do not like how he treated me when we broke up...

WHAT IS THIS GUY'S PLAN? He has never mentioned hanging out or hooking up again.... just a couple of brief hellos.