i.e., of the illegal stuff that goes through their system, some of it is captured, and the majority of that is caught by Facebook's own system.
Note, this article is recent and highly relevant: https://www.nytimes.com/interactive/2019/09/28/us/child-sex-...
> And when tech companies cooperate fully, encryption and anonymization can create digital hiding places for perpetrators. Facebook announced in March plans to encrypt Messenger, which last year was responsible for nearly 12 million of the 18.4 million worldwide reports of child sexual abuse material
> Data obtained through a public records request suggests Facebook’s plans to encrypt Messenger in the coming years will lead to vast numbers of images of child abuse going undetected. The data shows that WhatsApp, the company’s encrypted messaging app, submits only a small fraction of the reports Messenger does.