A few years ago after a US school shooting, the gun debate was everywhere, and I saw a 3 hour old anti-gun post from an acquaintance, on Facebook. It had 2 likes, and zero comments, the likes weren't from mutual friends. I clicked on her profile and saw a dozen posts of hers from the previous 12 months or so, many commented on multiple times, by mutual friends.
I hadn't seen any of them.
Just recently, same thing with an old friend from High School. She took a Covax selfie and posted it. No interaction from anyone, but there it is on my timeline.
Click her profile, and its full of random posts I've never been shown, with lots of engagement, from mutual friends.
We even have Facebook whistleblowers leaking internal policies.
If the service is free, the product is me.