I think I missed that they had refused to visit because they didn't like being told they were wrong.
You have bigger problems with them than Facebook, although that it itself is bad.
You asked them not to put it on Facebook. They ignored you and sent a group message to a large amount of people and then put a "bundle is here" comment on their home page for everybody. "Bundle is here" isn't exactly the enigma code.
So they are prepared to completely ignore you because it doesn't suit them.
And as much as they may have been excited to share the news, they should have understood that so were you. And they still took the opportunity for themselves and put their own feelings first.
And then they sulked and refused to visit the new baby they were so excited about. For what? To punish you both? To teach you a lesson? To make sure your DH doesn't call them out next time they ignore you and do as they please wit your baby?
I mean, I said before my in-laws were similar and this sulking is part of their pattern.
If they do something wrong to you on Monday, by Friday you've apologised to them for it.
Do people in your family spend a lot of time "keeping the peace" around your PILs by any chance?