Facebook’s mutual-friends feature concept has been in controversies in the past, raising privacy concerns. Facebook allows you to set visibility of your list of Facebook friends to “Only Me” if you want to keep your friends list hidden from other Facebook users as well as your own friends. Facebook CEO Mark Zuckerberg does not share his friends list with the public and neither I am on his friend list. Therefore, using Facebook Friends Mapper tool I would be able to see most of his hidden Facebook friends by exploiting the’mutual friend’ logical flaw.
Source: https://thehackernews.com/2015/05/facebook-friends-mapper.html