When people say Java, I almost always think about mobile and enterprise applications. While true you can build desktop apps with Java, that's not a common usage.

And, mind you, you forgot the platform almost every internet connected server runs.

But really, who uses C# on anything but Windows.

Rdio

Dow Jones

Tesco

Microsoft

Bosch-Siemens

Direct Energy

Ara

Sennheiser

Kimberley-Clark

Xactware

National Instruments

To pick just the people on http://xamarin.com/customers

A choice I'm sure they will regret in a few years...