Web 2.0 sites make up the fastest-growing category on the Web--doubling their traffic over the last year, according to data presented Monday by Nielsen//NetRatings at an industry conference in New York.

Web 2.0 sites - defined loosely in the session as those allowing users to "talk" to their "friends" via e-mail, messaging, blogs, and other social media tools--ranked first in year-over-year growth in unique audience and Web pages viewed. That put Web 2.0 ahead of categories including news and information, ISPs, video and movies, and family resources.

Social networking sites with the highest traffic growth included Feedburner (385%), Digg.com (286%), MySpace (170%), Wikipedia (161%), and Facebook (134%).

The Nielsen//NetRatings data also showed that engagement with Web 2.0 sites had grown over the last year, with retention rate increases of 10% at MySpace, 46% at Wikipedia and 20% at Facebook. Web 2.0 users also tend to be more active than typical Web users in online search, with 63.8 searches per month compared to 44.7 for the total market.

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