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.

IG should still be Django I would think? I'm kind of curious about Facebook / IG's use of GraphQL, is it hack or just nodejs?

You might be right about IG, I never worked on it so I’m not sure. Certainly there was still some Django when I joined in 2015 but now, who knows.

I’m not sure what you mean about the GraphQL question. Meta doesn’t use Node at all, or at least not to any significant extent.

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

Well “hack”, but the same idea