Chris Andersson, of The Long Tail fame, points to an analysis about how web 2.0 businesses actually make money. The biggest revenue seems to be online advertizing. I would have though biggest source of income is Venture Capital investments.
The original analysis (by Box UK) is both interesting and useful. It shows what has been tried, what is being used, what are related possibilities. For start-ups it can help to (re)define what is their market and business model. Usually one model is not enough, so it's useful to see options where to target next growth sprint.
Analysis is based on data from Webware's "Top 100 web apps for 2008". This gives an enourmous amount of credibility to the analysis. You can see several well know companies, who are actually using these models: Amazon, Google, iPhone, Opera, eBay, !Yahoo, Skype, Twitter, Wikipedia, Facebook, bitTorrent, YouTube.
Some are more successful than others. Something to seriously think about.