If it's any consolation, this is also a mystery to non developers like me. And developers likewise wonder why a business needs so many managers and what they do all day.
There is certainly room to add developers to a "simple" project if one is ensuring everything works with screen readers for the blind, that it had worldwide I18N support, meets every law around privacy and data jurisdiction, has systems for requesting personal copies/deletion, etc...
But instead it was probably for Messenger's portion of telemetry and marketing and ads and hacking out of your phones security model to spy on you. [0]
[0] https://www.eff.org/deeplinks/2025/06/protect-yourself-metas...
I bet one is enough.
Managers schedule meetings so that developers can’t get anything done. Then they hire more developers, which means even more meetings. Virtuous cycle!