One "reason" would be that FB derives the most income from the US, especially compared to the US. They certainly benefit from the US education system, which at least in part benefits from the infrastructure and social environment in the US that was created and paid for over a long period of time by essentially all Americans.

In the past, you wouldn't even have to ask this question, you would employ Americans as a matter of principle, as a matter of pride, to give back and reinforce this shared infrastructure and society for the benefit of future generations.

But then, none of this really matters in the new and improved globalist every man for himself world. (While the executives preach the virtues of socialist values.)

"especially compared to the the Philippines"