Nothing? Not even a clear conscience?
The "but someone else will do it" argument is used to justify so many wrong doings… We hear it all the time about fighting climate change, building unethical weapons, etc.
You also have the choice to not personally actively contribute to whatever unethical thing is being discussed. It's a circular game theory problem, that "other person that will do it anyway" might also be justifying it because they think you would do it if they didn't.
Serious question: Do you vote? Because this argument seems like people who say “my vote doesn’t matter, it’s just one vote out of millions.”
It’s true that at the individual level, one vote basically never makes the difference, but elections are won and lost. In the same way, if one person says no, maybe it will barely make a difference, but teams are made of more than one person.
I've worked in banking and my ex worked for Facebook, and I genuinely don't see the comparison beyond rather inaccurate tropes. Bankers aren't tearing apart society and sowing division like Facebook is.
Smart people know that you gain nothing by sacrificing yourself for society. Even if you cared enough to say no, all that would happen is that Facebook would pay somebody else to work for them.