"If your cv lists 'going to the pub' and nothing else under hobbies,you may not find yourself setting job short lists on fire any time soon,is this illegal discrimination due to 'chosen pastimes'? Similarly if your social media shows you to have no concept of personal discretion, expect the same thing, common sense surely?"
Um, no.
Or at least only in the sense that good sense and common sense often diverge.
Do you really think putting something on your CV for the consideration of a prospective employer is the same thing as talking about it elsewhere with your friends?
If you are telling people not to talk about stuff online that employers might find distasteful, you need to include things like whether they are disabled, whether they are a mother, whether they are of certain ethnic backgrounds.
If you are hiring for a publicly advertised job, you are not allowed to screen people out because you find their political views unpalatable.
You have to be able to show why you shortlisted the people you did.
Listing your searches and your personal objections to things they have said online will be interesting.
If you find out things you are specifically not allowed to even ask about at interview, how do you show that you didn't use that information as a reason to exclude?
You yourself said earlier that employers would (and should) use this method to screen out people who said things they found unpalatable.
Do you really think that is OK?
You can try to cover it with weasel words like "lack of discretion", but it amounts to the same thing.
A rugby-loving pisshead who makes sexist jokes will be AOK, but an outspoken disability campaigner will be suspect.
That's why we have equality legislation.
And that is why this is dodgy ground.