Hitwise put out some new data today showing that Web 2.0 sites (I didn't use the term so don't throw things at me) make up 12% of Web traffic, up from 2% two years ago. Here's a link to the PDF of a full presentation. Download file
...they also had some interesting data on participation rates at popular sites that backs up, yet again, the point that only a small percentage of folks actually are participating.
The researcher also points to Yelp, StumbleUpon, imeem, Veoh, WeeWorld and Piczo as the next potential breakout Web 2.0 sites.