I found your claim hard to believe so I went digging but couldn't find any recent resource on the total number of downloads npmjs.org handles per month.
I could have sworn they used to publish aggregate stats prominently on their home page somewhere ... but can't seem to find it.
Anyway, aggregating* the download numbers of just 12 of the top npm packages comes to about ~504.4m, or half a billion package downloads a month.
I was referring to the number of packages available as being more. It wouldn't be very useful to compare number of downloads since php does need an entire package to do things easily like check if something is an array.
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.
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.