I had been off facebook for 5 years. Now my kid started school and there's info in private facebook groups that is not shared any other way. I hate it but decided to rejoin.
So wait...the school uses Facebook for communication? Gross. My kids' school uses something called ClassDojo, which is exactly like Facebook, but geared towards classroom settings. It works well enough.
I'm guessing that parents connect through Facebook groups.
Today, being the first day of school here in Colorado, my wife told me that the parents decided to organize a Facebook group to share information. My suggestion was a Whatsapp group, but I guess that's Facebook as well :).