Background: this guy was my manager when I was at FB. My day was similar, something in the realm of 10:30 to 6:30—hours are flexible, but there’s some accountability for meetings, and you generally want to overlap as much as possible with your coworkers.
Typically I would deal with emails, do code review (you can review pretty much anybody’s code if you want), write new code, do the weekly or bi-weekly release if it was my turn to be on call, sometimes pair-program, and of course enjoy the free food and facilities (arcade, gym).
The software development flow (task management, tooling, reviews, testing, deployment, monitoring, etc.) is all very well polished, and I miss it at Microsoft.