Attorney General Eric Holder said the network, called Dreamboard, was under investigation for 20 months.
Authorities have now charged 72 people and arrested 52 of them.
Holder said the network was like a "members only" club, trading tens of thousands of images and videos of sexually abused children ages 12 and under.
"Dreamboard's members allegedly traded graphic images and video of adults molesting young children - often violent - and created a massive private library of images of child sexual abuse," said Holder.
Homeland Security Secretary Janet Napolitano said the suspected porn dealers operated in 14 countries.
Twenty remain at large. Officials said 13 people have pleaded guilty in the conspiracy and four have been sentenced.
