Social media

My boyfriend recently added this girl on Facebook, that he doesn't know. He wasn't talking to her or anything. He just simply added her. We've been together for three years, I feel there's no need for that. You don't need to add random people you don't know when your in a relationship. I don't feel he was trying to do anything wrong, he just didn't see anything wrong with it. He thinks I'm just being over dramatic with the whole thing, and says "it's Facebook". Like it shouldn't matter or be a big deal. 

I tried using another example as to saying, he shouldn't be making new random female friends in real life either. Not saying he can't have friends, but saying if he went to Wendy's to get lunch on his lunch break and some random girl came up to him and started talking to him and than give him her number, I wouldn't be cool with that.  

I feel once your in a relationship you make mutual friends, at least when it comes to the opposite sexy. 

So opinions ? 

Oh side note : he did end up deleting that random girl. Not so much because he was understanding where I was coming from, but because he didn't wanna fight.