What determines the success of a Web 2.0 site?

Recently, I chatted with my friends who are entrepreneurs and listened to their ideas about the projects. Their projects are based on Web 2.0 but actually do traditional businesses. Their growth means a great increase in employees, which also indicates that the market is expanding continuously. Besides, this process does not ensure profits, and it's very hard to estimate the break-even point for now. The most important thing is that maybe all these actions are not proportional to the development of the websites.