As another poster pointed out, you're assuming the reports are always unique, where in reality the number of unique instances/articles facebook has to investiga

As another poster pointed out, you're assuming the reports are always unique, where in reality the number of unique instances/articles facebook has to investigate is going to be less than the number of times people report something. Also 2 million isn't "the smallest number that counts as plural millions" - anything over 1 million is considered "millions". Lastly you seem to be assuming that this is 100% manual and that each report needs to be treated as seriously as each other report. I'd imagine that at the very least these reports will be prioritized and if something is reported once by a user who reports 100 things a week, it will probably be ignored - giving more time for higher priority things.