For one thing, Mr. Keen says, “history has proven that the crowd is not often very wise,” embracing unwise ideas like “slavery, infanticide, George W. Bush’s war in Iraq, Britney Spears.” The crowd created the tech bubble of the 1990s, just as it created the disastrous Tulipmania that swept the Netherlands in the 17th century.

Mr. Keen also points out that Google search results — which answer “search queries not with what is most true or most reliable, but merely what is most popular” — can be manipulated by “Google bombing” (which “involves simply linking a large number of sites to a certain page” to “raise the ranking of any given site in Google’s search results”). And he cites a recent Wall Street Journal article reporting that hot lists on social networking Web sites are often shaped by a small number of users: that at Digg.com, which has 900,000 registered users, 30 people were responsible at one point for submitting one-third of the postings on the home page; and at Netscape.com, a single user was behind 217 stories over a two-week period, or 13 percent of all stories that reached the most popular list in that period.

Because Web 2.0 celebrates the “noble amateur” over the expert, and because many search engines and Web sites tout popularity rather than reliability, Mr. Keen notes, it’s easy for misinformation and rumors to proliferate in cyberspace. For instance, the online encyclopedia Wikipedia (which relies upon volunteer editors and contributors) gets way more traffic than the Web site run by Encyclopedia Britannica (which relies upon experts and scholars), even though the interactive format employed by Wikipedia opens it to postings that are inaccurate, unverified, even downright fraudulent. This year it was revealed that a contributor using the name Essjay, who had edited thousands of Wikipedia articles and was once one of the few people given the authority to arbitrate disputes between writers, was a 24-year-old named Ryan Jordan, not the tenured professor he claimed to be.

Image Credit... Tony Cenicola/The New York Times

Since contributors to Wikipedia and YouTube are frequently anonymous, it’s hard for users to be certain of their identity — or their agendas. Postings about political candidates, for instance, can be made by opponents disguising their motives; and propaganda can be passed off as news or information. For that matter, as Mr. Keen points out, the idea of objectivity is becoming increasingly passé in the relativistic realm of the Web, where bloggers cherry-pick information and promote speculation and spin as fact. Whereas historians and journalists traditionally strived to deliver the best available truth possible, many bloggers revel in their own subjectivity, and many Web 2.0 users simply use the Net, in Mr. Keen’s words, to confirm their “own partisan views and link to others with the same ideologies.” What’s more, as mutually agreed upon facts become more elusive, informed debate about important social and political issues of the day becomes more difficult as well.