Facebook had to develop their own PHP-to-C++ transpiler to achieve something vaguely resembling useful performance. I wouldn't quote that as positive example…
I just don't understand PHP bashers always using Facebook as a metric to bash PHP. "Oh PHP is so horrible, the second (sometimes even first ranked) most visited site on the internet had to write their own PHP-to-C++ transpiler to handle the load. Clearly PHP isn't a good choice for my 3 user site! I need that performance!"
Get over yourselves already. Every language has it's strengths and weaknesses.
Yes, Facebook had to write their own interpreter, but you also know what? Facebook also actively helps the PHP community and is ushering the community too. So anything Facebook creates they open source and PHP7 has taken cues from them. The whole net benefits from it.
Not to mention the next biggest website that is powered by PHP is Wikipedia, whom uses stock PHP to serve their sites.
Facebook has so much interactions and that, that they warrant the need for what they did. 99.9999% of most people don't need that. Not even Wikipedia. They just render a html page. Done.