[brownra04]

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.