Ruby projects in general appear to scale toward the JVM. Both Twitter and Tumblr began as Ruby projects but now make extensive use of Scala. The JVM gives you access to powerful new tools like Hadoop and the various distributed databases and search utilities.
Ruby tends to find the most use on Macs and Linux-based systems. Thus you want an option that scales to those systems' strengths. You say Mono? Mono could work but is still fundamentally an immature ecosystem compared to the JVM. Particularly regarding language support outside C#
So why then Clojure? Clojure helps smooth some of the problems that large distributed projects face. Functional languages tend to scale better with large sets of concurrent data than Object Oriented languages do. It's also easier to move from Ruby to a Lisp than it is to move to Scala which is more like an ML (SML, OCaml, Haskell, etc). You could use JRuby but again, it's still going to suffer some of the limitations of a pure OO language.
Lastly, Ruby is mostly popular as a web language because of Rails. Yes, there are other options but Rails is the overwhelming majority in terms of deployment. Using Clojure gives you access to all manner of mature web solutions, being designed to function in that environment from the get go rather than adopted into that role 10 years after initial release.