Instagram is struggling to stay on top of a secret network of child pornography as its 15,000 moderators shared with Facebook try to police the dark side of the social media network which hit 1 billion monthly users last July. A ring of pedophiles sharing sickening images are using the app and desktop-based service to lead people to less-obviously advertised links to endless content on Dropbox. The file hosting service is unknowingly allowing pedophiles to circulate indecent images of underage children.