If proof were needed of how much faster and smaller the world is as a result of social media, it comes in the form of this story. Without the social news network Reddit, it seems likely James Erwin would still be writing software manuals in Des Moines, Iowa.

Instead, he is writing a Hollywood movie treatment. The process began when he answered one of the hypothetical questions people often post on Reddit. In this case the user calling himself "The_Quiet_Earth" asked essentially how a group of time-traveling U.S. Marines would...