Which Web 2.0 sites are the most commonly used?

As you think about your Web 2.0 strategy, one question you should ask is: Which sites are the most commonly used? A good place to answer the first question is Compete. I did a search today to get a feel for the traffic on Twitter. From 12/07 - 12/08, the traffic has increased by 752.9%!!! I'm one of those people who had some doubts about using Twitter, but based on that traffic, I need to reconsider!