, have you asked him (NOT when either of you are angry, upset or tired) what he gets out of having her as his friend on FB? I am, by no means, agreeing that it is OK that he does. Personally, I think it's a cr*p idea. It's just that I'd guess it has become an issue for him. That by keeping her there, he's showing you that you can't tell him what to do.
So don't follow up his likely response of "Nothing really" with "Then why do it when you know it hurts me?". Just say "OK" in an accepting and neutral tone. And carry on doing your thing. I repeat, do not ask him why he does it when he knows it hurts you. Just leave it. Maybe he has a reason (sceptical look), maybe even a good reason (even more sceptical look). The point is to make him think about why he's doing it. I have (not that long ago - sigh) realised after starting a fight with my husband for no particularly good reason, that it was actually only because I wanted his FULL attention on ME. Seriously - how old am I?
It strikes me that you are both stuck in a never-ending loop of hurt. One of you hurt the other, then got hurt back, so did more hurting. And so on.
Do you believe that he is committed to your marriage? Do you genuinely believe that he isn't having an affair, or wanting to? Do you believe that he loves you?
I'd suggest that maybe you're not angry about her, you're angry about him appearing not to care that he hurt you with his ill-chosen actions. But I'd challenge you by asking how often you tell him that you appreciate those 1000 things that he does do right? And that isn't "I appreciate x, BUT why haven't you also done y?". Rather it's "Thank you, I appreciate x, it makes me feel loved/chosen/special/cherished/looked after/like we're really good for each other." End of conversation. Carry on doing your thing.
You said that you'd tried MC - how many different therapists have you tried? Sometimes a certain approach clicks, and sometimes it does not. At all. I've had a therapist who annoyed me hugely with her sickly sweetness ... which, now that I think about it, was probably very effective in getting me "healed". I've had another where I've had a huge breakthrough in understanding myself in one session. He was a man of very few words. Which weren't mine that he was just playing back to me.
Then this a much harder piece of advice to give. The reality is that you've hurt each other badly. Getting love to grow in a hostile environment is really hard. Unless you're both committed to having something more than what you've got now, you're never going to move forward together. If he isn't willing to be part of this with you, then you need to decide what you need and want.
We keep telling ourselves that these people who keep hurting us really love us, and have so many good qualities. Reality is always a harder story to tell ourselves, so we sometimes opt for the easier story.