Out of curiosity, what sort of things would bring down a site as big as Facebook like this? I'm sure they have many checks, balances, and methods of ensuring a

Out of curiosity, what sort of things would bring down a site as big as Facebook like this? I'm sure they have many checks, balances, and methods of ensuring a code change wouldn't impact the site (to this extent) - and they are distributed across many data-centers, so I wouldn't think that would cause it to go down.