Yes, you should feel angry. Another MIL unable to behave decently.
You sound like you've been through quite a bit yourself recently. Pregnant with a young child, in a new country and with a husband who is away a lot. I think you've handled it all pretty well. She sounds awful.
The facebook thing is irrelevant, its just the tip of the iceberg really, I know exactly what you mean about stuff like that. I too had a good relationship with my MIL on some levels but in the end she just doesn't operate on the same level. I think in a way, for her, its all a game.
I made a decision around 18 months ago to disconnect from my MIL - emotionally speaking. I just stopped thinking about her at all, in fact her and FIL. I have absolutely no control over what she thinks or says about me. Her behaviour, the things she says and does, bear no relation to what I have done or what I do so there was just no point in banging my head against a wall.
It really is the best thing I have done. It has been so freeing. I have a great relationship with all of my own family and I really put 110% into the relationship with my IL's. They fucked it up again and again (10 years) Eventually you have to take a stand and say - no, enough is enough. You are not going to be the source of stress and conflict in my life.
My advice is to disconnect and leave it to your husband to conduct a relationship with his own family. I don't put any restrictions at all on visits but I make it clear that I'm not willing to visit the IL's house (after a number of huge, loud nasty rows between them all and conducted in front of my kids)Funnily enough, since I took a step back, my husband makes no effort and as a result we rarely see her.
On the rare occassions she does come through, I practice the killing with kindness approach. To be honest, I don't really know how else to be, I can't do the bitch thing and silence and moods just stress me out so I'm just myself.
I've gone through the letter writing, the heart to heart etc etc but in the end, the only honesty was coming from me. Eventually, as hard as it is to get to grips with, you have to realise that some people are just toxic, they will never ever think or act in truthful, honest and decent way. They are always going to look for disharmony and problems and you will never ever get to the root of it.
My MIL always had a toxic relationship with my husband too, he just wasn't aware of it. You really have to ask yourself what kind of a mother she is when she wants to be the source of conflict and stress in her newly married son's life.
Good luck - having been in a similar position to you many times over, I empathise, the stress is extraordinary. The stopping thinking about her takes practice but only a couple of weeks and I was cured.