Technically facebook does not use PHP. They used to use PHP a long time ago. They have had a long run on ongoing development to compile php to c++. Their most recent (as far as I know off) is a JIT compiler named hiphop. Also, facebook uses a lot of c++ and other technologies as well. Saying facebook uses PHP for maintaining their internal systems is even not overstatement, its simply not true. I love PHP (as well) don't get me wrong. But use tools for what they are good at. It is never a good idea to build an infrastructure on PHP. Use it for microservices, use it for consumer applications. You might use it for trivial backend processes that don't need to scale. But dont try to use PHP as a one stop solution. It will cause headaches. It will slow down internal processes and it will reclaim the development time you saved by using php in the first place and it will happen long before you'll be like facebook.
Perhaps I should add that my statement should not be a real issue for most websites. Just don't expect PHP to be your friend when you are dealing with millions of requests, or background processes dealing with gigabytes or tens of gigabytes. Especially when you have to deal with different development teams that rely on eachothers processes. :')