I think its wrong and sneeky tbh.
OK, you maybe shouldn't do it, but you could verbally tell all your friends, who also tell theirs and word gets about very quickly. You don't need fb to damage a company and this should be taken into account in situations like this. they don't know how few or many actually read your post, but likewise you could have inadvertently informed 100 people just by telling a mate.
It seems they can do this quite legally if they want rid of you for any reason. it could be a work colleague who actually reported seeing this btw.
With my ds1 they were taking the data protection route, which was stupid as he hadn't leaked any data protected or otherwise.
He was just moaning about his lot, a bit.
Look out for another job, just in case. But be prepared for the worst, it does sound like somebody has been waiting to drop you in it.
As you aren't fitting into their work schedule for them, as your OP, it wouldn't surprise me if they don't want rid of you because the hours you want are difficult for them.
At least find a representative to take with you and listen to what they say. If it looks like you are going to be pushed resign for Gods sake.
A new employer won't think twice if you tell them how unsupportive they were being after your mat break