I don't think it's that people are stupid. Blindly trusting these apps to not take advantage or mishandle their access to personal data is also naive.

It's possible to do business without scanning your customer's faces, tracing their contacts, selling details of their transactions to aggregators, etc ... where does the line get drawn?

And that doesn't even address data that is "accidentally" collected.

People may not be stupid but they are extremely gullible and they also love hating on some perceived bad guy or corporation.

And yet after a whatsapp call I'll be advertised things I'd never mentioned by any other medium. It is striking.

Nah. You think that you have never mentioned or searched those things, but in reality you're just starting to recognize them as soon as your call ends because your brain has been primed to pay more attention.

This is the same phenomenon as driving with other people and suddenly asking out loud "Hey let's find such or such type of car". Whatever the make (within reason), you'll probably start finding out a bunch of them. That's because you start paying attention, of course, not because there's suddenly a surge of people driving them at the exact time you're looking for them.

There are other ways to accomplish that without a microphone. Hint: there was another party on your call other than yourself, and their behavior (on the web for example, where you may be linked as friends on the same social media sites) can provide signals you are unaware of, that suggest things about their friends including you.

This should be easy to prove by reverse engineering the WhatsApp application.

If someone could come up with proof of this, they'd probably be set for life in one way or another. Perhaps they could monetize the information by collaborating with short-sellers, or simply kickstart a career using the massive media attention.

WhatsApp call are end-to-end encrypted. Do you think they aren’t? (All, only yours?)