Well “hack”, but the same idea

The entire website is still a PHP-like derivative language (with an optimizing compiler, etc). Backend services are largely C++.

If you mean stock vanilla php: none. If you mean Hack, Meta’s PHP dialect which is internally just called “PHP” colloquially: a lot. Basically everything that responds to a public HTTP request, both for APIs and for the website. And since Meta is a lot less “service-oriented” than some companies, a lot more application logic happens in the web server process than you might expect.

Backend services in languages other than Hack do exist, of course. When I left Meta (then called Facebook) in 2019, they were almost exclusively in C++. Now I don’t know for sure but I think Rust is gaining a lot of popularity for non-Hack stuff.