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Tagging as it is used at some of the Web's most interesting and lively new sites is launching a revolution of self-organization on the Internet. You could call it the latest twist in the ongoing evolution of social networking software. Except there's a difference: On social networking sites like Orkut or Friendster, people join, and then declare their alliances to each other explicitly. On sites that employ tagging, the networks emerge, implicitly, out of the shared interests of users. Order isn't proclaimed, it just happens.