I just had an interesting experience the other day.
We recently moved to a new place and my partner was looking for furniture from her phone. She uses a standard Android device with Google Play and everything, but she uses MaterialFBook from fdroid to join Facebook (less intrusive).
A couple of days after the move, I started getting lots of targeted furniture ads in Facebook myself, specifically from the websites she was browsing or bought stuff from.
I suppose either some FB tracker picked up our IP, or companies upload their client’s details (including IP) to FaceBook to increase their reach.