“It is hard to imagine that five years ago, neither YouTube, Facebook nor Twitter existed. But even then, as sites like Google, Amazon, Wikipedia and craigslist flourished, the characteristics common to successful second-generation Web businesses were becoming apparent: Their value was facilitated by software and created collectively by and for a community of connected users. These sites leveraged the Web not simply as a means to publish static documents but for the first time as a platform–which was significant in its generative properties as the personal computer was for desktop applications. The new sites also sparked a revolution in business, culture, society and, most recently, government.”

Read more:

http://www.forbes.com/2009/09/23/web-squared-oreilly-technology-breakthroughs-web2point0.html