The figures emerged as seven countries published a statement warning of the impact of end-to-end encryption on public safety online.
Facebook has previously announced plans to fully encrypt communications in its Messenger app, as well as its Instagram Direct service – on top of WhatsApp, which is already encrypted – meaning no-one apart from the sender and recipient can read or modify messages.
The social network said the changes are designed to improve user privacy on all of its platforms.
But law enforcement agencies fear the move will have a devastating impact on their ability to target pedophiles and protect children online.
Last year, some 16.9 million referrals were made by US tech firms to the National Centre for Missing and Exploited Children, including 69 million images of children being abused – up 50% on the previous year.
Some 94% of the reports, which include the worst category of images, came from Facebook.
But the National Crime Agency has warned the number could drop to zero if Facebook presses ahead with end-to-end encryption.
But NCA director Robert Jones said of the plan: “The lights go out, the door gets slammed, and we lose all of that insight. It is as simple as that.”