What are some of Facebook's best kept secrets?

Not sure these are "best kept" or even "secrets," but I'll give it a shot.

1. Nobody has an office, not even Zuck. Some execs have their own assigned meeting rooms, but their actual working desks are with everyone else's.
2.  Scaling is a way of life. When I joined, the company had around 400ish people and around 50m users. 4 years later, about 4000 employees (10x) and 1B users (20x). If that's not scaling, I don't know what is.
3. Facebook Groups is the main way teams at Facebook collaborate and share information. It's probably one of the most heavily used Facebook products by Facebook employees. I love it.
4. Facebookers tend to take their work seriously but don't take themselves seriously. In my experience, those are the best kind of people to work with.