I get jealous when my bf mentions/likes girls on social media?

This really depends on who he's interacting with and how. For instance liking his female friends pictures is normal, liking pictures of random girls with their cleavage bulging out isn't. Tweeting/ retweeting neutral/amusing things with friends is normal, commenting "Damn, so gorgeous *string of emojis* isn't."

We need more info before we can make a judgement.

You need to talk with him about your insecurities.

I think it's normal to feel the way that you do. If he knows people, especially girls, that you don't know, and he is liking their pictures, that is bound to bring out your anxiety and feelings of insecurity. I had to leave my husband because he kept going on all sorts of adult sites and creating multiple profiles to talk to other women. Unfortunately, the more you complain or show that you are bothered by it, the more they will hate you for it. It sucks. :(

This is another reason why social media can be a waste of time and energy.

when i opened my bf's fb, i saw that he liked some pictures of his girl friends but never liked any of my pictures. I never told him I know about it, it will only backfire on me. Social media issue is a toxic in a relationship.