C#’s a weird one: it’s widely used, but still unpopular. It’s got genuinely large problems (like the anemic OS ecosystem) but when considered as a language on its own, it’s been ahead of the game in its class.
Mainstream uses for .NET are windows desktop software (WPF & XAML are the best GUI toolkits for the platform), videogames (Unity3D is quite popular), cross platform mobile development (Xamarin).
Now when .NET Core supports ARM Linux, it also works well for some embedded applications, but I don’t know whether somewhere except me does that.