First, social networks are attractive to pedophiles and a number of other "problem" users because they simply provide the opportunity to anonymously connect with like-minded people. Many child pornography collectors have obsessive traits. How many news stories have you read about someone getting caught with only a few child pornography images? Generally, it's a matter of hundreds or even thousands. Facebook is one of the biggest players in the social networking arena and it also offers features like image and video sharing and private groups. So it wasn't a matter of if pedophiles would show up on Facebook, it was a matter of when.
Facebook initially seemed to focus on growing and adding new technological features and may not have anticipated that it would also attract child pornography traders until they ended up with quite a few of them. The problem was that by that time, there were millions of accounts. So they had to come up with automated ways of finding users who did not want to be found. Automatically shutting down accounts annoyed the people behind them, but they could always just create new profiles. So many of them did. To make things even more interesting, the problem users came from a number of countries, some of which are a lot more proactive about fighting child pornography than others.
For the most part, the pedophiles trading child pornography on Facebook seemed pretty stupid -- not too far removed from the idiots sharing it on open P2P networks. The ones on Darknet seemed smarter and scarier. I'd challenge the WND writers and anyone else interested in fighting child pornography to investigate that and try to come up with viable solutions.