Exactly. Flickr and Tumblr are both primarily about the content - there's no need to force users to login before they can explore. Also Facebook, Twitter et. a

Exactly. Flickr and Tumblr are both primarily about the content - there's no need to force users to login before they can explore. Also Facebook, Twitter et. al are the wrong examples to be looking at - brands already so strong people will jump through hoops to use them. If you're Yahoo! - coming from behind - your brand and even the brands of Flickr and Tumblr aren't strong enough to to require users login before they get to try anything. In the case of Tumblr you might get the _existing_ userbase from the web to come with you but you won't succeed in growing that audience on mobile. Examples that do use gradual engagement would be games. Clash of the Clans for example - a huge success on mobile - let's you immediately start playing without any form of identification. Many mobile games have strong social elements but they focus first on getting you engaged before requiring anything.