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