We have a product that has a C# web app deployed via mono on a linux box commercially shipping for four years. What I found was that with mono 2.10.6 it was fine as long as it was not loaded heavily, but under stress it would have stability issues. The socket layer was rewritten since then however and I've had less issues with 2.10.8/9 but still fails under heavy load. More fixes have gone into mono 3. My sense is that it's getting better but not nearly as battle tested compared to JVM-based network stacks. So if I was coding a large scale web service I'd consider Scala first.