I would feel so violated if I accidentally left very personal handwritten notes in a library book that I returned, then have the library scan it and very publicly turn it into social media fodder. These notes are not for any of us to read.
[1]: https://www.ala.org/tools/ethics [2]: https://www.ifla.org/publications/ifla-code-of-ethics-for-li...
Granted, I'm also not a big personal social media guy in general, so maybe that's just my bias peeking through.
Of course, the physical book will soon be a thing of the past at the public library (-_-) so I suppose these considerations will soon be moot.