Nir Eyal's 2013 book Hooked completely changed the way people think about creating viral technology and products.

The apps that won, Eyal said, did so because they tapped into something deep in the brain. Facebook, Farmville, Angry Bird, Foursquare—like hit songs, these apps wormed their way into our heads. They made us come back again and again. These companies were all successful at one thing; creating a hook that formed strong user habits.