This is in the sense of largest number of end users
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millions use TiKV (adopters), a distributed transactional key-value database.
Here are some numbers for some of those adopters:
- over 200m users (late 2020)
- over 220m users (mid 2019)
- over 300m users (mid 2019)
This is, for me, the most impressive Rust success story.
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over 700m people use Dropbox (late 2022), whose file storage system is written in Rust.
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over 200m people use Firefox (late 2022), which has various components written in Rust, most notably Stylo, which does CSS style calculation.
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250m people use Discord (early 2020), which has Read States service written in Rust, whose "sole purpose is to keep track of which channels and messages you have read".
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100s of millions use Android 11+, whose DNS-over-HTTP/3 service is written in Rust. A number of other components are written using the language, especially starting with Android 13.