Honestly, I would tell C's girlfriend that he's dating people behind her back and if she knows anything about it. It forces the situation to be out in the open because that kind of behavior just bugs me.

It sounds like he just wanted a fling. You didn't necessarily know that and M didn't seem to pass that information along either. He's insecure enough that he wants attention from females and arrogant enough to think whoever he's with will never pick up on it. He's immature and dumb, to be really honest with you. It's pretty clear to me that he's not invested in finding an actual relationship, which it seems like you're trying to find. I think your instinct to delete him from your Facebook list is spot on.

I have no idea why M is involved at all and she does seem awful pushy about it. I would want to ask her why the heck she's getting so pushy about a guy who she presumably isn't even friends with. I can't honestly think of why she would (unless she's just the overeager type?), but I don't necessarily jump to her being in some sort of relationshi with C. She could be, I have no idea.