Was this true for the systems that related to revenue and ad sales as well? While I can believe that a lot of code at Facebook goes into production without first going through a staging environment, I would be extremely surprised if the same were true for their ads systems or anything that dealt with payment flows.
I worked on ad systems at Facebook, and yes it's (approximately) true for those as well.
The thing to realize that "in production" almost never means "rolled out to 100% of users from 0%". Instead you'd do very slow rollouts to, say, 1% of people in Hungary (or whatever) and use a ton of automatic measurements over time as well as lots and lots of tests to validate that things were working as expected before rolling out a little more, then a little more after that. By the time the code is actually being hit by the majority of users, it's often been run billions of times already.
I don't know about Facebook, but at other companies without similar, each git branch gets deployed to its own subdomain, so manual testing etc. can happen prior to a merge. Dangerous changes are feature flagged or gated as much as possible to allow prod feedback after merge before enabling the changes for everyone.
