It's from a November 2019 report by Indeed of 0.1% of job postings mentioning Rust (https://www.hiringlab.org/2019/11/19/todays-top-tech-skills/), with growth of 44% compared to 2018 (https://www.hiringlab.org/2019/11/26/fastest-rising-tech-ski...).

But you can even have a rough estimate according to your own measure. There are roughly between 50K and 100K developers working at FAANG (maybe replace Netflix with Microsoft). How many of them would you guess are using Rust?