Is Facebook still coded in PHP?
It started as a pure PHP codebase.
Then Facebook became “viral” and old PHP could not cope with millions of concurrent requests per second.
The company worked in two directions:
1) Create it’s own high performance version of PHP running on a proprietary, optimized virtual machine. The Facebook PHP flavour has been called “Hack” and the virtual machine HHVM.
The new generation PHP 7 actually includes a lot of the ideas for improvements that Facebook introduced in their own version. Nowadays, developing a proprietary version of PHP would be almost redundant: PHP 7+ took most of the good ideas and made them its own.