"""This claim is clearly false on Mac OS X and X Window platforms, and disputable on Windows, with the increasing share of .NET apps."""
Actually it is not.
On OS X tons of programs, and especially the very largest, most popular ones, are in C++ (think: Photoshop and the Creative Suite, MS Office, and lots of Apple's own: Safari (webkit) is C++, Final Cut and Logic are C++, etc. Don't confuse the front-end UI code (Cocoa) with the backend. C++ is a first class language in XCode, and you can mix it with Obj-C as Objective-C++.
On X Window platforms: besides KDE, some of the most important apps are C++, including Firefox, Thunderbird and Open Office.
On Windows there are almost no .NET apps that people actually use --people as end users, as opposed to corporations. Most of the popular stuff, from games, to office suites, to design suites, to browsers, etc are C++.
