Wired has a pretty good piece on a game that started as a tweet. Its tracks the organic nature of this particular experience . . . as some games tend to grow. A developer with a pretty small following posted a tweet about making a game: "Contemplating building a game entirely with friends on twitter/fb. Totally open and ‘Mad Lib’ style. Could be fun or totally awful."
With that Mike Mika started a ball rolling. But it wasn't until another developer from another company commented that the ball got started rolling.
Give it a read, its worth it. Link above.