Can you provide any evidence that personal data doesn't improve CTR prediction for companies like Google/Facebook?
You state yourself that Google/Facebook publicly claim to advertisers that personal data improves CTR prediction. So I have a hard time believing that personal data isn't useful.
I’m already on a shaky limb being so candid about how the business actually works. If you want the opinion (albeit a little dated but still relevant) of someone who doesn’t give a fuck about who the truth pisses off I recommend a book called “Chaos Monkeys” written by a former YC (exited) founder.
You state yourself that Google/Facebook publicly claim to advertisers that personal data improves CTR prediction. So I have a hard time believing that personal data isn't useful.