Several components of the Dropbox core file-storage system were written in Rust as one step in part of a larger project to pursue greater datacenter efficiency. It’s currently used by all Dropbox storage today, serving >500 million users.
Yelp has developed a framework in Rust for real-time A/B testing. It’s used across all Yelp websites and apps, and experiment subjects range from UX to internal infrastructure. Rust was chosen because it’s as fast as C (cheap to run) and safer than C (cheap to maintain).