I'm not appalled that someone wouldn't know who Paul McCartney is as much as I'm flummoxed that they would ask the question on social media. Better to keep your mouth shut and be thought a fool than open it and remove all doubt, to paraphrase Abe Lincoln and/or Mark Twain and/or the book of Proverbs.
A collaboration between McCartney and chart-topping hip hop artist Kanye West caused a stir on Twitter when a number of insular dullards praised Kanye for giving the "unknown" old geezer some exposure. Evidently, the one thing Kanye's fans and the lost tribes of the Amazon have in common is that Beatlemania and its enduring legacy passed them by. Not that they should be worshiping at the altar of the erstwhile Fab Four - after all, there's an enormous chronological and artistic gulf between Black Skinhead and Ob-La-Di, Ob-La-Da - but it takes a committed ignoramus to have never heard of Paul McCartney and the Beatles.
This is the Information Age. No matter how much of a personal bubble you live in, access to answers is immediate, and knowledge - no matter how trivial or irrelevant - is difficult to avoid. If a 72 year old is collaborating with an entertainer of Kanye West's stature and notoriety, it stands to reason that the old guy has some pedigree. A quick Google search could save you a world of embarrassment and ridicule.
That is, assuming you've heard of Google.