Web 2.0 can be broken down into three major types of sites. The first is news type sites like Digg, reddit and Newsvine.
The second popular type of site is the blog type sites like MySpace, Nexopia and Facebook.
The third major type of Web 2.0 sites are Social bookmarking sites like delicious and blogmarks as well as many others.
The above page reviews all of the major players for social network bookmarking and is really great if you are wondering about the relative size of all of these services and the extra features that each offer. The leaders according to the site are:
Traffic wise Stumble upon is a great site as well.