I asked about this and some Rust folks were kind enough to mention that Dropbox is one company that publicly has a large Rust deployment. Further, I think the main (huge?) advantage of Rust is it was designed to allow in situ replacements of pieces of a large c++ codebase. This really is a killer feature for lots of folks who simply can't afford to rewrite tens of millions of lines of tested c++.
previous discussion: https://news.ycombinator.com/item?id=10890310
Over the years several languages have been the 'C++ killer'. For quite a long time it was D, and I know several people who converted. Then the standard library split in two, and the language got fairly unusable for a while. Now things have calmed down with D 2.0, but it's not clear it will still be around in 10-15 years time.
Similarly with Rust -- at the moment it seems to be a one company product (Mozilla). What if they get bored of it? Will my Rust code still work in 10 years time?