The Devalue and Discard stage that occurs when divorcing a NARC is the absolute worst. It's the worst because it's the nastiest most overt attack a narcissist can concoct. Compared with the daily, sneaky barrage of belittling and criticisms and such, his behavior becomes outright and overt emotional and verbal abuse. And the result makes you feel like you were in an accident. The shock is just unreal. A narcissist will strike at you using Facebook, or any means for that matter. I sometimes wonder why they bother to do this, and I suspect they think there is some way to hurt us worse than they have, or that they are punishing us somehow.
But rest assured, eventually you begin to see his machinations and strategies for what they are, and that you can pull their teeth before they bite you. He doesn't know it. He may never know it. Let's be real: he thinks he doesn't need improvement, but you on the other hand probably just barely passed muster, right? He will try the same manipulative nonsense over and over, but once you have his number, there is nowhere to go except forward. And always remember the kids are of utmost importance, but beware that he may try to use them as a lever or weapon against you, and that will damage them.
He will attempt to manipulate their opinions and feelings, at their expense, for his own purposes. A narcissist always comes first in his own mind. Don't do FB. Block him. Don't look. Just don't. Expect to obsess in your head for some time over him and the acrimony of the not-yet-settled-because-of-him divorce is STILL dragging on. Also expect that the obsessing, which hurts like hell, is merely your mind attempting to make sense of the trauma; trying to heal itself; trying to understand something a normal person has little chance of understanding. It takes time.
Expect to become very angry. As in enraged. My friend of 50 years got me a "dammit doll" and I hate to tell you how many times that thing got slammed on the counter, or how often I sobbed on the phone to her while I was beating the stuffing out of the doll. Eventually, even if you obsess, it doesn't hurt as much. And as you move forward, and educate yourself more and more and work in therapy, you begin to see that YOU are making progress. You will get stronger. Yep, at first it's slow and nasty and depressing and you can't even drag yourself out of bed, but keep going, because you will survive and you will turn that corner. You might not even notice, but you will. One day you will think about him, and you may still be in disbelief, still hurt, still lonely and so forth, but he simply cannot wriggle into your head any more. You have realized what he never will.