When Apple first announced Swift, iOS developers across LinkedIn started learning it. This deck by Software Engineers Kyle Sherman and Kamilah Taylor describes LinkedIn's experience with writing 2 drastically different apps in Swift.
The SlideShare app has a much smaller user base, number of developers, and fewer features. The new LinkedIn app, Project Voyager, is a very large codebase with a large group of developers and a huge user base. This should give you the idea of what type of challenges and benefits both a startup and large company would have by switching to Swift from Objective-C.
This presentation was given at the Sillicon Valley iOS Developers' Meetup on 12/07/2015. Thanks to Tim Burks for hosting and organizing the event.