There’s a lot of buzz these days about viral loops, and how critical they can be to the success of certain startups.  It’s one thing to understand the concept in theory.  It’s quite another to see miniature case studies of successful user flows laid out in an easy to consume manner.

That’s exactly what the new and incredibly useful ProductPlanner does.  Consider Facebook’s Sign Up loop, for instance.  Once you’ve walked through the user flow in isolation, the elements that make the product so damn viral quickly become clear.

Put in terms of the finite state machine I mentioned, Facebook makes it a priority to locate all other existing users who might possibly be friends with the new user.  The app leverages Metcalfe’s Law to provide an immediate and exponential increase in value, hastening the transition from new user to casual or even power user.

But the real kicker, is even in a worst case scenario where the new user fails to find value in the service and immediately jumps to the dormant user state, he provides some value to Facebook since new users promote the service even as they sign up!