I recently had an idea for a story that included a sub-plot that involved the serial murders of members of a social forum not unlike Gransnet, although there are some distinct and important dissimilarities. To check whether it is possible to identify people on a social forum where they use only screen names I asked a friend to see if he could track down six names picked at random without any special software and, most importantly, without breaking the law.
He traced all six names, addresses, numbers of children, workplaces and bank accounts. In a couple of cases he traced hospital treatments and appointments. He backtracked to schools, colleges and universities but didn't have time to check all of them. This was done in his spare time in a week.
Obviously, if my character is a member of this imaginary forum, he/she will become au fait with many aspects of the lives of other posters.
I don't think there is any such thing as privacy any more.
Btw The random names didn't come from gransnet.