Because they have way too much power over content.
This week they suddenly, and without any explanation, closed the FB page of The Shade Room. The Shade Room, apparently, is a celebrity gossip/news site. I have never visited but I learn it has earned more than 4.4 million likes; and the owner/manager of the site, Angie Nwandu, was listed as one of Forbes’ 30 Under 30 In Media this year. Ms Nwandu reports that she has been targeted before by Facebook.
Because no explanation has been forthcoming, one can theorize that one of Zuckerberg’s Hollywood pals didn’t like something The Shade Room wrote and he agreed to censor them without further ado.
Other Reasons NOT to use Facebook.