Wow. This is great stuff. This post is a must read:

When done right, gradual engagement communicates the core essence of a service with a few lightweight interactions. If you can make people successful along the way-even better. Will Wright, the creator of the Sims & Spore,  has a belief that games should allow people to succeed within the first five seconds. That’s a great philosophy to bring to gradual engagement. In fact, I think if you can use lightweight actions to allow people to accomplish something relevant to the core of your product within their first one or two interactions with your service, that’s gradual engagement at its finest.

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Through their  user research, Twitter found that while celebrities (and their tweets) were a big reason people came to Twitter, they did not keep them there. Instead, what kept users on Twitter was the things they were passionate about – hobbies, conversations with subject-matter experts, and friends. This was the core essence of the service that a gradual engagement approach needed to deliver.