Teachers cannot negotiate salary, thanks to unions. The best teacher in America is paid the same as the worst teacher.

And no every union is about teachers, which are by defenition not comparable to white collar tech employees (coming back to the FB example).

Grade teachers on their students' test scores? Okay, now you have teachers spending inordinate amounts of time on overfitting the curriculum to the local standardized testing requirements and less time actually ensuring their students are educated.

Grade developers on their lines of code or number of features shipped? Be prepared for needlessly verbose code and buggy garbage shipped out to meet the metric.

Throw these objective criterion out the window in favor of a more judgment-based approach? Be prepared for lawsuits when the mistake is eventually made.