A guy I'm talking to posted on FB that he went out with another girl?!?!?

At no time does it say in here that he is committed to you as a boyfriend. Sounds like you are assuming he is your boyfriend and that he does not think of you as his girlfriend. It sounds like all he was trying to do is get you to have sex with him. I would not make a big deal out of it, just do not trust him or pursue him. I am glad you had the sense not to sleep with him. Just because a guy is nice to you and does a few things with you does not mean you are special to him. Something like that takes time. The way to know how true, caring and honest a guy is to you is to save it for marriage.