For more than 10 years, Google has organized the Web by figuring out who has authority. The company measures which sites have the most links pointing to them—crucial votes of confidence—and checks to see whether a site grew to prominence slowly and organically, which tends to be a marker of quality. If a site amasses a zillion links overnight, it's almost certainly spam.
But the real-time Web behaves in the opposite fashion. It's all about "trending topics"—zOMG a plane crash!—which by their very nature generate a massive number of links and postings within minutes. And a search engine can't spend days deciding what is the most crucial site or posting; people want to know immediately.
So a new generation of search engines like Tweetmeme, OneRiot, Topsy, Scoopler, and Collecta are trying to redefine what makes a piece of information important.