Facebook recently became the principal sponsor of the Python Software Foundation[1]. Although I don't have any concrete numbers to cite, there's a lot of Python code at Facebook. Calling it a PHP shop would be highly inaccurate and myopic.

[1]: https://www.python.org/psf/sponsorship/sponsors/

Disclaimer: I work there on python runtime optimization, which mostly involves writing C (and C++) code. Also, I've never had to write any PHP/Hack code.