"Here's something you probably don't know about the Internet: Simply by designing your product the right way, you can build a billion-dollar business from scratch. No advertising or marketing budget, no need for a sales force, and venture capitalists will kill for the chance to throw money at you.
The secret is what's called a "viral expansion loop," a concept little known outside of Silicon Valley (go ahead, Google it -- you won't find much). It's a type of engineering alchemy that, done right, almost guarantees a self-replicating, borglike growth: One user becomes two, then four, eight, to a million and beyond. It's not unlike taking a penny and doubling it daily for 30 days. By the end of a week, you'd have 64 cents; within two weeks, $81.92; by day 30, about $5.4 million.Viral loops have emerged as perhaps the most significant business accelerant to hit Silicon Valley since the search engine. They power many of the icons of Web 2.0, including , PayPal, YouTube, , Facebook, MySpace, Digg, LinkedIn, Twitter, and Flickr..."
Full article at Fast Company.