Have you seen the current thread comparing the SDMB to reddit?
One point made there is that one’s experience with reddit can vary quite a bit depending on which subreddits one frequents.
This is at least a little bit true of Facebook as well. What you encounter on Facebook will depend on who your friends are and what they post, and on what (if any) groups you’ve joined or people you follow. My Facebook isn’t “a right-wing conspiracy theory haven,” but certainly it could be that for other people.