Cathy there are millions of people in the U.K. but they aren’t all on MN.
MN is self selecting but it’s demographics is (mostly) women with children. So that cuts out millions and millions of people.
Posting personal info cuts it down again.
Eg. I have twins. Only 1 in every 80 births are twins so again that dramatically cuts down the numbers of potential people.
My twins are 10 years old. So that narrows it down much further.
I have boy girl twins which cuts it down again.
So now we aren’t taking about millions anymore, we’re possibly talking about thousands.
So if over the course of a few threads I happen to mention that I live in a particular county, or mention eg that my DD is a talented gymnast, DS plays grade 8 piano, I’m a red head and that we went to Paris at half term suddenly it becomes really easy for someone who knows the family to confirm that it’s us.
I’d still be “anonymous” to most people but not to anyone who thought they recognised a scenario and decided to spend five minutes checking.
People don’t deliberately give away lots of personal details but if you are on MN for years without name changing a quick advanced search picks up lots and lots of info.
People regularly contribute to threads giving their age, their car, their profession, describing their homes, their tattoos etc etc etc.
If you name change regularly you might still be identified but at least they can’t trawl through years of posts not all of which you might want discussed at the school gate.
MN isn’t private. It isn’t really anonymous.