Nudity is common and natural at any age in many locations in the world. Its mostly the western cultures such as Americunts where its taboo and criminalized. Many people do post nudes and even home made pornography on Facebook but usually set the rights to access to certain person or people. And sometimes there is user error that makes access available to more than intended and also those who understand how access rights work can sometimes hack access. Facebook has improved security overtime though so its much harder today then it was in 2003.
There was a time not that long ago the the mid sequence of numbers on any Facebook picture is all you needed to access that persons photo pages if you knew how to do it. You did not need a user name, just that sequence of numbers preceded by the proper URL. Today they use URL timestamps and can no longer be done but I am sure some already have FORBIDDENovered a work around.
Nothing on the internet is ever 100% secure from those with skills.