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There are perceptual image hashes[0]. There even are databases that have "search by similar image" indexing[1]

[0] https://www.phash.org/ [1] http://www.lire-project.net/

Neural nets, more exactly, convolutional neural nets. They are quite effective nowadays.

Example: SphereFace: Deep Hypersphere Embedding for Face Recognition

To be fair the type of people who are sharing naked photos via Facebook aren't likely to be all that technologically sophisticated to enough understand hashing.

Pedophiles would be using something encrypted and anonymous anyway.

> Pedophiles would be using something encrypted and anonymous anyway.

Except for the ones who apply for a job reviewing the nudes Facebook is collecting, anyway.