"The third is the one that we are very interested in," Mr. Weiner said. This is "social" or "community" searching, in which each attempt to find the right restaurant listing, medical advice site, vacation tip or other bit of information takes advantage of other people's successes and failures in locating the same information.

The idea that human judgment can improve a search engine's automatic findings is hardly new. From the dawn of the Web's history -- that is, over the last 15 years -- companies have invented tools to help users assess the quality and relevance of information, often by relying on others' opinions. Examples include Amazon's user reviews, eBay's feedback ratings and "trusted networks" created on many sites.

What is different is Yahoo's systematic plan to build "community intelligence" into nearly all aspects of its operation -- and in turn, to entice users to spend more and more of their time on Yahoo sites, where they can see Yahoo ads. The clearest example, of many I heard about, can be seen at http://myweb2.search.yahoo.com, the beta version of a new search site.

A query from this page will return results from three sources. One is "My Web," or pages each user has marked and asked Yahoo to save for later reference. (These pages are saved by Yahoo itself, on its servers, and don't gum up your own machine.) Another is "Everyone's Web," the general Internet. Finally, there is "My Community's Web," pages marked as interesting or valuable by members of a social network. Thus, a search for information on new cars would bring up normal Web results, but also listings you had seen and wanted to retain, as well as friends' advice on brands and dealers they had tried.

Setting up a social network to provide advice can take time. But Caterina Fake, one of the founders of Flickr and now a Yahoo executive, pointed out that virtually everyone under 30 had already created such networks. What about those not young or hip enough to have done so yet? Eventually, according to Ms. Fake, more users would create networks as the process became easier and more worthwhile. Mr. Nazem said, "We're really about getting the average consumer to move their lives online."

Why should Yahoo's community intelligence be better than others' half-successful earlier attempts? This is where its argument about scale comes in. "It is a key strength that our community is so large," said Mr. Semel, who has seen Yahoo's user base double in his four years as chief executive. With hundreds of millions of users, there is critical mass to create social networks that cover most locations and interests -- for instance, a large and active user group among women in the United Arab Emirates.

VAST scale presents its challenges. "You can think about the way people will interact, as you sit in the usability lab, but until you put it in front of very large numbers of real people, you don't really know," Ms. Fake said. "So you have to release products early and often, like perpetual beta."