I am close with some people who worked there until recently. All data access is audited; production access is limited via ACLs in both the main data storage system as well as all the others like the warehouse, realtime ingestion, etc.

FB appears to take this extremely seriously. I just pinged my friends and they said the only way people get fired is for sexual harassment or improper data access. And the second is the one that gets audited and monitored every day.