Facebook and the like throw off billions upon billions in profit and the market encouraged companies to spend for growth.

There isn't really an opportunity cost to hiring more people when money is no object. More likely to be the opposite.

Why do you think they increased their headcount so much in the last few years? Simply because they could.

If you go a read a FAANG annual report, you’ll find the headcounts are in fact finite, meaning they do in fact devote a finite budget to those resources. Just because the headcount or the budget seems so large that you can’t easily conceptualize it, doesn’t mean they are unlimited. Regardless of their motives for increasing headcounts, doing so has an opportunity cost, and so does the way in which they choose to do it.

What is the opportunity cost then? What are they trading off by hiring more people?

The fact there is finite budget and headcount doesn't necessarily mean anything. If you're limited by the number of quality employees you can recruit and onboard, budget and headcount are effectively meaningless because they're not limited by money.

How many useless projects does Google already waste money on? Google has already shown a complete ineptitude at coming out with new profit generating products. Throwing more people at problem with a broken culture won’t do any good.

That is an argument that they've reached a point where the cost of increasing engineering headcount is as high or higher than the return, not that you don't want to keep increasing headcount until you get there.