Viral Loop: From Facebook to Twitter, How Today’s Smartest Businesses Grow Themselves. It tells the stories of the fastest growing companies in history–Skype, Hotmail, eBay, PayPal, Facebook, Twitter, Flickr, and many more, all of which grew virally. By amassing such huge numbers of users without spending a dime on marketing, they were able to create multimillion and in some cases billion-dollar businesses practically overnight. They did it by creating a product that its users spread for them. In other words, to use it, they had to spread it.