But do people assume the same thing about, say, Google search results? Promoted posts on Reddit? The ads (or natural posts) on Snapchat or Instagram?

I agree that it's pretty obvious you're being retargeted when ads for camping supplies start showing up three days after you search for them on Amazon. But the practice of "personalization" of results and ads is far larger and deeper, to a degree that most people never seem to think about.

Agreed. Although we all hate it, if my mom searches for hard sided luggage on amazon and ads for it follow her to all manner of other sites - that’s the best way for non-tech types to get some of the idea here.

The truth is WAY worse of course, but she immediately knows the ads she saw won’t show for me as well.