This is the most important point here. Facebook can afford to do this because their users don't care. No one really needs Facebook or depends on it to work at critical times. Most products are not like that. Imagine what happened if Gmail started doing move fast and break things. Oops some of your email vanished.
On the contrary, I have to believe they do care. Many here might not remember or know, but early on, Facebook was very unstable due to the sheer server load. Performance is still an important metric for them. It is why they created HHVM, Hack, and React.js.
How often do you hear of Facebook going down or breaking completely?
