@Doctor SyntaxRe: 'It's not clear whether he also has a FB account or whether he's a non-account'
Sorry but from a technical aspect, there is no defense.
Regardless of your status as a member or not, FB captures and performs work on the data in order to build a profile. Its not until the later stages that they are able to match this information against a FB user.
Think of it this way.
You use Dr. Syntax here.
You may have your favorite fetish site where you go by igor
Your real name may be Christopher Robbins and on FB you go by the ailas Chris McDougal.
(I don't know I'm just making this example up ...)
So even if they can't match Christopher Robbins to a FB user, there is still data on you and it has value.
What they do with it is a mystery and under GPDR, its still illegal because they didn't get an explicit , informed, consent.
FB really doesn't have a strong leg to stand on in either case.