There's no reason to believe that as a job seeker you can have some insight for a given company into how impactful your actions may or may not have on the things they are doing that you consider harmful. An honest assessment I'd argue should assume your impact will be minimal if not zero given a large enough organization, unless you are coming in as a high level manager.
As such it's hard to not take it as a self-rationalization given the incentives to create cognitive dissonance on the part of job seekers in order to take a high paying job working on interesting technical problems despite the potential negative effects it has on others. This doesn't mean there aren't good reasons to go work at such a place, it just means that if you're telling yourself that you're able to look past any ethical concerns you might have since you think you can be an influencer towards correcting them, I think it's worth reflecting on if you're not actually going to be put into a real position of power to do so.
The challenge with Facebook specifically on this front is that power is extremely centralized onto one person, so even if you were joining Facebook as, say, a VP, it's hard to know in advance just how much influence you're going to have over the direction of the company before you've learned where the boundaries of that influence are.