His commenting on your fb, in combination with messaging you and the way he is doing it, is entirely due to your professing that you're attracted to him. In my opinion his behavior while mild, is a yellow warning flag about him. It speaks volumes that he's taking the time to comment and engage with you when he already has a gf. Why? Because his whole attention is based on the fact that you're already attracted to him and it says to me that he, in spite of being involved is toying with other women in this way. Flirting off hand occasionally happens but this is intentional and purposefully done.

Best case scenario he's just enjoying your attention and means nothing by it but even so that's entirely unfair to you and is really an attempt to keep you on a string. Worst case, he's the type that now or eventually he will probably cheat on his gf and is that really the kind of guy you want to associate with?

My first wife, I met as she approached me and gave me her number, we were both young. I called her and then she mentioned she already has a bf. I told her straight up, "call me if you're single again" That was a mistake because she did, we ended up together, and pretty much the entire marriage of about 6 yrs, she cheated on me repeatedly before I finally had the solid evidence on it and called her out.

I only add my experience because people that are devoted and committed dont' play games with others that happen to have an attraction to them.