Facebook tends to be one of the highest payers in industry. It's possible paying even higher would net more people, but overall math suggests the best N people won't necessary be US citizens or permanent residents.

The law doesn't say you can hire the best person if they are abroad though.

It effectively says you can't hire a better person from abroad if there is a person who is good-enough domestically.

(I'm only saying what the law says, not what is best for Facebook, the USA, or even what is morally right)

"qualified" is completely arbitrary. I can just set the threshold high enough such that there aren't enough "qualified" people domestically.