I maintain the GNUstep / Clang Objective-C stack. Most people who use it now do so in Android applications. A lot of popular apps have a core in Objective-C with the Foundation framework (sometimes they use GNUstep's on Android, more often they'll use one of the proprietary versions that includes code from libFoundation, GNUstep and Cocotron, but they almost all use clang and the GNUstep Objective-C runtime). Amusingly, there are actually more devices deployed with my Objective-C stack than Apple's. The advantage for developers is that their core logic is portable everywhere, but the GUIs can be in Objective-C with UIKit on iOS or Java on Android (or, commonly for games, GLES with a little tiny bit of platform-specific setup code). I suspect that one of the big reasons why the app situation on Windows Phone sucks is that you can't do this with a Windows port.
It would be great for these people to have an open source Swift that integrated cleanly with open source Objective-C stacks. Let's not forget that that's exactly what Swift is: a higher-level language designed for dealing with Objective-C libraries (not specifically Apple libraries).
Objective-C is a good language for mid-1990s development. Swift looks like a nice language for early 2000s development. Hopefully someone will come up with a good language for late 2010s development soon...