I'm not a step-parent so I could be way off base, but have you thought about approaching him and asking him about the Facebook status update?
You could start off with something like "Hey Zach, I know I'm not your mom and I'm not trying to be, but I wanted to ask you about something I saw a little while back on Facebook that hurt my feelings."

Since it will be impossible to be a parental figure to him, be a friend. Friends usually speak honestly and openly with one another. If you treat him with respect, I'm sure he'll reciprocate in time.

I have a step-mother but my Dad did not remarry until I was in my early 20's. I actually had no idea he was even dating anyone as I was living in a completely different country at the time.
Our relationship was strained from the beginning and I remember being angry with her for changing my home. Suddenly my childhood home didn't feel like home anymore. They painted it, added new furniture etc. and I felt like a stranger in a place I should have felt most at home.
Our relationship came to the breaking point at Christmas that first year and I overheard her talking to my grandmother about my "entitled" behaviour. (I didn'T want to watch Scrooge Christmas morning, as we had watched the movie the night before, so I went on the computer to check my e-mail which was apparently a no-no.) I flipped out on her and left and I didn't speak to her or my dad for a long time.

Among other small issues, the biggest issue I had with her is that she didn't treat me like an adult or with respect.
This is why I suggested you approach him and speak with him openly about the way he spoke of you on Facebook.

BTW, my step-mother and I are now good friends LOL