Social media presents all sorts of problems for schools in terms of confidentiality. So, the example of discussing a near abduction might wander into discussing the child in question. Or even discussing what's happened to your own child isn't really fair on your child - they might not want you to talk about them to their friend's parents.
The other issue is it can be used as a form of bullying. Wouldn't moaning about a particular teacher when they have no way of defending themselves be a form of harassment?
Also, it can be very cliquey. Some parents are in the group, others are not. Some aren't on FB at all. So it should not become the main source of information.
Also, it's pretty pointless. Why complain about school proceedure etc on social media when the school might not hear about it? Nothing will be done. Pissing into the wind. Better to take your concerns to the school. Or join the governors board. Or join the parent teachers association. Do something practical to help. Be kind and positive rather than spread negativity.
Yes, I do work in a school but not a teacher. I would be gutted if someone was discussing my professionalism on social media and I had no idea or was unable to defend myself. Not that I'd want to on social media.
Honestly, it's sad that HT have to ask parents to respect the school and it's staff and children this way. To point out what people should naturally know is not right. There are avenues for complaints. If the school isn't responding adequately, you go to the board of governors. If they don't respond, go above them to the local council.