First off OP you said snooking and it made me giggle.
Secondly I would just let it go. It sounds like he could care less. He didn't even accept her friend request, you did. Maybe he sensed she had a thing for him? Idk I wouldn't let it bother me and try to move on. My DH wouldn't tell me if someone poked him because to him it's just not important and he would not be interested in poking anyone back.
i honestly wouldn't care, i used to have major trust issues, but being with my husband he doesn't give me a reason not to trust him, he's not the best on keeping up with things, but even if i find things out i just ask him about it and he'll explain it. getting over trust issues does take a lot of time to get over, but being single doesn't always help, being with that one person that will prove to you that he can be an honest and faithful guy helps
I really think you are over reacting in this situation. I wouldnt be impressed that the chick did that especially if shes not talking to me on facebook and befriending us both. But if he's not responding and not doing anything you cant give him crap about it. Even if he didnt tell you about the stupid poke thing he probably just didnt think it was that big of deal and left it. If he repsonds to her and never told you that they are actually talking then I would wonder but he's really not doing anything wrong.