Facebook employee here. A significant chunk of onboarding is driving the message through that if you abuse your powers as an engineer to access personal user data, you will be insta-fired without warning. There are a number of checks in place to ensure that employees that don't need access to the data for their specific work cannot get it either.

Additionally, the privacy review process for new products/features is extremely stringent and time consuming. Product teams have been hamstrung significantly, with some engineers I know complaining that they're not even able to get half as much work done as they used to.