I think these are all good questions. They're certainly things I mulled over when considering whether to take the job.
Regarding being fundamentally evil or not, I've worked around groups which were constitutionally amoral or immoral-- the mandate of the group was simply incompatible with common-sense notions of good and little effort was made to reconcile the two. Facebook does not seem to belong in that camp, but does belong in the grey wasteland of groups that are well paid to develop a peculiarly self-serving sense of ethics. The reason that's important is because amoral groups will simply not accept ethical arguments as valid arguments to make, while groups with peculiar ethics are responsive to them but sometimes their logical toolkit is filled with double-ended hammers and crescent screwdrivers. I've found that to be an important distinction in previous roles.
As you point out, there is an ethical hazard here. If I worked there for five years would I find their ethical blind spots so obvious? Is the environment corrosive in a spiritual sense? No idea, but early indications both during my interviews and speaking to current employees were mixed. It was definitely something that gave me pause.
In terms of whether you can actually have the requisite influence, I probably would have taken the job if I could have come to confidence on this. In the end I don't think I could have and I didn't take the role.
Of course, it's difficult to disentangle the compensation from the levelling, so I may just be lying to myself and saying that if they'd paid me more I would have pretended. To attempt to guard against that I asked friends and former colleagues who I thought knew both me and something about the dilemma and listened to their thoughts. It's not a perfect system but I think it yielded a set of thought experiments that were useful to me in making sure I didn't fall into that trap.