i just reregistered briefly to say thank you to Carmen and others who tried to help.
By clicking on that fb link when I was logged in (I suppose I was curious about what would happen and thought it might take me to the mn group or something) I've created an undeletable piece of electronic history, a record that links my anonymous persona on here to the real me. It's not just on my pc. I never had a link before, I never shared, never linked to any other site, it was all at maximum privacy. No personal data on there to be picked up and shared.
We all know how loose fb's respect for privacy is: it's already found my school and university though I've never even searched them on there. Nothing I can do now changes that link I've unwittingly created, no matter how many cookies I delete, or forums I deregister from.
I don't like that, much in the way I don't want Tescos emailing me when my period is due because Tampax are on special offer. Yes I was stupid to click on it, and I didn't realise it would make that pathway. It's not exactly made clear. I didn't know that would happen because I'm no expert, I'm just usually pretty careful.
So anyway, thanks to those with helpful suggestions. Bye.