Where do book lovers hangout on the Internet?

They don't hang out on the internet, they are reading books :D

More serious answer: Goodreads.

I am making a small website that would be useful for book readers. Where can I promote my product?

Second Goodreads... Kindle integration is a big plus!

Still Goodreads, but damn, that site is ugly. But this is where my community is hanging, so...

(From my personal experience) I've visited:

Nowdays Facebook Groups took over (mostly due to their convenience and ease of use).

Goodreads, Amazon, and Audible are the only book related sites I visit.

Producr Hunt is a good place to show/promote book related websites! I had mediocre succes with where i got almost 100 paid signups via product hunt. I am also working on a telegram community but havent posted it on ProductHunt yet,

naval ravikant's twitter