We had a problem in a closed group on Facebook recently concerning the teaching and homework/kids not writing what the homework was in their books etc. in my child's year. It rapidly turned into nastiness.
So I split on them and gave the school a heads up, and told the group if they have any problems to stop moaning and take it to the teachers to get it sorted out rather than talking behind their backs.
One of the teachers has an account on Facebook for putting reminders and year info out so he put a tactful message out about not hesitating to contact them if there are problems etc. they are happy to help. Also, homework had been given out a day late because of a year trip so he used that to tactfully introduce the subject of homework, where it is written down and that the children are to take responsibility for writing down what they are to do for homework in allotted time in the mornings.
Yes, discuss things on Facebook. If your child has written the wrong page number down for their homework ask the parents so the teachers don't have to answer half the parents asking the what the homework is, but don't criticise the teachers' teaching, head teachers decisions about who does assemblies etc. when you should be asking the school itself.
Also, there was a lockdown in a secondary school in our area today. All sorts of rumours were being spouted on Facebook and people started criticising the area, it's going downhill etc. etc. Within 3 hours people were having to ask others to watch what they were saying so lies were not put out there and so panic didn't set in. (It was asserted someone who should not have been on site was wondering around school with a metal bar, far from the truth.)
We need to be careful what we say/put on social media, it just takes one wrong sentence to cause major problems.