I'll admit I don't know a ton about how much this would have affected it but could it be because FB/Meta was not part of the salary fixing ring (Apple, Google, Intel, and Adobe) so they were paying "market" rates vs the fixed ones.

I'm a big fan of increasing developer compensation, but if that compensation is based on inflated and unsustainable business models/goals, then it's a false promise.

Meta is certainly not the first tech company I'd accuse of having an unsustainable business model, they actually built a money printing machine and aren't getting devs on illusory future revenue streams.