At a recent Facebook tech talk at their headquarters, that company revealed that almost every day, Facebook engineers push hundreds of bug fixes, new features, product improvements, etc.; some big, some little. Most of the 600 million-plus users never notice a thing.
As Facebook engineer Chuck Rossi details around minute 23:00 in the video below, Facebook has a tool they call "Gatekeeper" which allows them to be in control of who can see what code live on the service at any given time. As Rossi points out, right now on Facebook.com there is already the code for every major thing Facebook is going to launch in the next six months and beyond!
The whole talk is in the video below: