Very few people are using it at all, outside of calling OSX/iOS APIs. They're using it to weld platform support into a C++ codebase, same way they'd use ObjC for that (instead of writing entire apps in it).
The number of pure-Swift projects I've seen per tempus is far less than pure-ObjC, which I think is the best measurement to comprehend Swift uptake. Nobody wanted an ObjC replacement, Apple misread the room.