Here's something you may not know about today's Internet. Simply by designing your product the right way, you can build a flourishing business from scratch. No advertising or marketing budget, no need for a sales force, and venture capitalists will flock to throw money at you. Many of the most successful Web 2.0 companies, including MySpace, YouTube, eBay, and rising stars like Twitter and Flickr, are prime examples of what journalist Adam L. Penenberg calls a “viral loop”—to use it, you have to spread it. After all, what's the sense of being on Facebook if none of your friends are The result: Never before has there been the potential to create wealth this fast, on this scale, and starting with so little. In this game-changing must-read, Penenberg tells the fascinating story of the entrepreneurs who first harnessed the unprecedented potential of viral loops to create the successful online businesses—some worth billions of…