Not surprisingly email is top on the list.  Searching for specific information is second. From Neilson Online.

Genre Impressions
Email
9,337,597,000
Portals & Search Engines
6,489,517,000
General Community
5,987,675,000
General/National News
3,204,711,000
Finance
3,204,123,000
Entertainment
1,896,307,000
Sports & Recreation
1,344,017,000
Local/Regional
1,289,481,000
Weather
876,198,000
Shopping & Auction
796,058,000
Other
4,584,728,000
Total
39,010,412,000