My own mother gave me a bit of advice a few years after I married. She said "Let your husband handle his parents".
It is difficult to sit back and see a spouse being emotionally mistreated by bullying parents. You think being supportive is getting involved, giving your two cents, and telling your spouse how he should handle it. Actually, by doing all those things, the only thing you will accomplish is a divorce.
Your spouse looks to you to be supportive....an outlet from the insanity and someone who understands. Your job is to be there if he wants to talk, but not to tell him what you would do. Just listen. That is all you should do. Just say "You did what you felt was best". Never respond with "I told you this was what you had to do".
Never initiate conversation about his parents. To you, they don't even exist. Use that advice. Your spouse doesn't want to hear you asking him what she has said lately. He doesn't want to hear you telling him he should call her and "put her in her place once and for all". Don't even ask about her. You will find he will be more at ease and better capable of handling it the way he feels most comfortable. He has to do this without feeling like he is doing what you told him to do.
Just remember that he is trying to save his parental relationship. Nobody wants to cut off their own parents. You are causing tension whether you realize it or not. He wants to make everyone happy. He is the one who should ultimately be happy and he isn't right now. He has mom attacking him and you are coming at him from the other direction. He feels trapped. I am sure right now he feels like he has chosen you over his mother. Afterall, his response toward mother is what you felt should have happened. Mom probably feels rejected whether she is at fault or not. She has responded the only way she knows how. Yes, it might be childish to take you off Facebook, but you don't need to add fuel to the fire by telling your husband that. Just leave it alone. Don't even talk about her or her actions. Let him handle it.