Hi SingleDad,
I just want to lend my support to you. It's very good that you tried to stay on point and calm, and you stopped talking when things began to escalate.
It's a fact that most people at your son's age, and even older post stupid pictures of themselves drinking, smoking, etc. In their world, exhibitionism is a given, and this can be problematic when they decide to search for a grown-up job, etc. They truly don't know what's wrong with this FB activity, as everyone does it, along with the partying lifestyle. If he drives a car, you have every right to be concerned.
Ultimately, he is an adult being given the privilege of living in your home. I think you need to make sure he knows you love him, and are only interested in what will be best for him in the long run. You are acting like a parent, not a buddy, and he's probably in shock, because hardly anyone does that anymore. Even if you did overreact, and he only does this stuff once in a while, he's keeping good grades, working, etc., he will know that you have limits and perhaps will recognize that it's time to grow up and act like a responsible man. I think you're doing the best you can in a tough situation. FWIW, my husband would have buried his head in the sand and acted like it never happened, and would hope it would just "go away" on its own!