I wonder in what form they store what the article calls a hash. Certainly no simple file hash - I guess it's some kind of feature extraction that lets them matc

I wonder in what form they store what the article calls a hash. Certainly no simple file hash - I guess it's some kind of feature extraction that lets them match the image against others? Facebook probably is even just reusing the same systems that flag child porn or other illegal images. One of their better and more user-friendly ideas for sure (if you are locked into their ecosystem anyway, you might as well use their surveillance expertise to control the content you spread there.)