Not in the same league as the mp4 parser shipping to all Firefox users, but GeckoDriver [1], a Mozilla-authored standalone binary for interacting with Firefox via the WebDriver protocol (e.g. using Selenium) is also written in Rust and shipping, possibly to as many as hundreds of users ;)
Overall the experience of using Rust for that project has been pretty great; the original requirements for a language were "able to produce static executables with no external runtime requirement, not prone to C-style memory safety issues, and accessible to a team with mostly Python backgrounds". That leaves quite a few options — Go for example — but being a Mozilla project taking the opportunity to use Rust seemed like it would align better with initiatives like those discussed in the article.
Apart from the general niceness of Rust-the-langauge, the experience of the Rust ecosystem has been really nice. Not only were there easy-to-install, well documented, packages to do much of the heavy lifting (http server, command line argument handling, etc.), it was also quite straightforward to set up travis builds using cargo to compile releases for linux64+musl osx and cross-compile releases for linux-arm7hf (added because someone asked about running on raspberry pi and it turned out to be trivial) and win64. The only wrinkle so far has been the difficulty of cross compiling to win32, meaning we might actually need to set up Appveyor or similar.
Obviously this is a much smaller project than the original post in terms of the number of users, and rather different as it is much higher level and could comfortably be written in Python or similar, except for the distribution requirements. However I think the use of Rust has been an unmitigated success, and I would be tempted to use it for a lot more projects that I would never consider writing in C/C++.
[1] https://github.com/mozilla/geckodriver