Years ago, in my middling tech explorations, I setup an Apache webserver to privately serve my music collection. I shared the link and password with a longtime friend over Instant Messaging. As she browsed my collection, I watched the server logs and commented through IM on what she was looking at.
She eventually told me to stop because it was creepy that I was watching her trawl through my collection to find something she likes. Of course, I stopped watching the logs and let her go about her business.
And yet log-watching is exactly what every recommendation algorithm does today.
It’s surprising how differently we act online when we know someone is taking notes.