Facebook users can protect their privacy by setting the privacy level for every information related their profile or content they post online. Users can decide to completely set as private the information in a way it results invisible to other users, even to the users friends. Even if the user decides to make the friends list invisible anyone can see it. Free Chrome extension called Facebook Friends Mapper can exploit the flaw to allow to view the hidden friends list in just one click”]
Source: https://securityaffairs.co/wordpress/36675/hacking/facebook-friends-mapper.html