Two weeks ago, Facebook launched their Facebook Pages iOS app. And last week, Facebook launched their Facebook Camera app.
One of the interesting aspects of the two applications is that their welcome screens greet you with the following pages: blue screen, big get started button, and (in the Facebook Camera example) a greeting specifically for me ("Continue as Ryan Spoon").
The Pages example is easy to explain: press "log in" and Facebook authenticates the user via the core Facebook Application already installed on the device. Easy. And of course an user of the Pages app will already have the Facebook app... it's easy to be presumptuous when you have Facebook's reach / scale.
The Camera app is more interesting - and the first time I have seen an example like this. It is also something only someone like Facebook can do (few others have that reach). It is remarkably fast, efficient, cool.... and effective - no worry about conversions, funnels, etc.
How do they do it? Here's a Quora post explaining: