Comment by Jtsummers

> No but whatsapp was built by 2 guys that had previously worked at Yahoo, and they picked a very strong tech for the backend: erlang.

https://news.ycombinator.com/item?id=44911553

Started as PHP, not as Erlang.

> 1) knew what it would take, 2) chose already from the ground up a good technology to have a smoother transition to your "X millions users".

No, as above, that was a pivot. They did not start from the ground up with Erlang or ejabberd, they adopted that later.