How are lower-level workers supposed to influence a crank like Zuckerberg?

He owns the company and all of its decisions. He is literally accountable to no one, from the board down.

The only viable action here is to do what the Brazilian government are going to do - document the abuses, and fine the company into oblivion and/or ban it if it doesn't behave itself.

If the US ever recovers democracy itself, pass the documentation on to the new US government.

How? If you don’t follow orders you get fired eventually. No one has any real power at Meta other than Zuck.

At least you're being honest about the fact that you're selling out. Many people working there believe that they're making the world a better place, or think that Meta is just another amoral company like the many others, or that they as an individual aren't contributing to the negative impacts. Money will do strange things to good people.