>They have basically forked their entire backend stack, starting from the OS and moving upwards.

I doubt this being a cool thing. Sure, they are the second biggest IT employer in the country, and have excess resources for everything, but...

A thing about big dotcoms - architects there try to use off-the-shelf software for everything, even if the software is clearly unsuited for the task and its use will require hacks and massive re-engineering.

Putting efforts to use of off-the-shelf software without modifications and hacks greatly reduces all aspects of infrastructure support burden.

BUT, attempting to use of off-the-shelf software everywhere at all costs, that will of course give you problems. And the bigger your are, the worse this is. Read the article from a month ago how Alibaba got stuck with using MySQL for mission critical tasks, and how much efforts they put to "unhack" it