We're having millions of unique clients per month and our peak traffic is above 200k requests per minute. Ruby delivers, although you need to be very careful what you deploy to the main app codebase. Especially to realize the speed differences between stuff like uniq vs. uniq! or the cost of creating new objects. It's enormous with big traffic.

There are some things I wouldn't do with Ruby here. Like some concurrent background jobs; better solution would be Clojure, Erlang or any language where the concurrency constructs are better thought and easier to manage. Although we're having threaded Ruby running, it's not very elegant and you can do pretty nasty bugs in there.