If companies like Facebook and Amazon had such a hard time hiring engineers, they wouldn't be cutting 10% of the workforce every year based on mostly political yearly reviews. That's why they are always constantly "hiring". Having visa workers that can be easily overworked is a huge bonus to try and force 80 hour work weeks and a toxic rat race.

The average engineer at these companies is not all that competent, the interview process is very noisy. I know many people barely able to write basic stuff like fizzbuzz, lots of them slither their way into middle or upper management.