To be fair, FB struggled to scale with PHP, although it certainly fares better out of the box (even then). A lot of their work has gone into making PHP more performant and better by way of Hack.
PHP7 took a lot of great ideas from Hack but is still saddled with its legacy.
They did? I remember numerous Twitter outages due to scale and the infamous Fail Whale. Twitter's issues risked the company losing users or even it's sustainability as a business.