Millions of citizens around the globe contribute to an online encyclopedia for free. Hundreds of young people show up simultaneously in a city square to happily lick ice cream. Computer programmers from Sweden to Silicon Valley collaborate online to build an operating system that outdoes even Microsoft -- none of them pocketing a penny. What do these things have in common?

According to Clay Shirky, a communications professor at New York University, they're examples of group activity that grew out of new communication tools -- from wikis to websites. And we will see more of it. In his extraordinarily perceptive new book, "Here Comes Everybody," he argues that this newly harnessed group power will change the way we work, achieving efficiencies and results far beyond the power of conventional organizations.

This is a compelling premise, and there are echoes of digital futurism in some of Shirky's bold declarations. But he goes much further than most writers on the topic of the Internet. Drawing from anthropology, economic theory and keen observation, he makes a strong case that new communications tools are making once-impossible forms of group action possible.

Shirky's range of examples is exciting. In Egypt, for example, dissidents communicate over blogs and a new online tool, Twitter, to alert others when friends are arrested or borders closed down. On Coney Island, the annual Mermaid Parade has taken on a life of its own because of the photos shared on Flickr. In New York City, outraged citizens combine forces on Digg, a collaborative news website, to get a woman's stolen Sidekick cell phone returned.

These are, of course, cherry-picked examples from the blizzard of talk and cant that fills up the Internet -- something Shirky readily admits. In fact, he believes the high preponderance of failure -- whether it is a blog about bunions, or Microsoft's own (not free) encyclopedia -- is one of the Internet's great strengths in marshaling group behavior. It allows people to try out ideas and participate in activities that would be too expensive for a company to direct or manage.

As powerful as these tools are, Shirky may be underplaying some of the possibly negative consequences: Will our social skills suffer from less person-to-person contact? And what about our acquisition of knowledge? Everyone knows that an old, printed encyclopedia might have out-of-date information. But what if you click on Wikipedia at the moment someone has sabotaged it?

Finally, Shirky is needlessly hard on newspapers (says the newspaper reviewer). Indeed, he rightly argues that newspapers were far too slow to grasp the Internet's power. But a newspaper couldn't exist without exactly the kind of group effort Shirky describes: Community members tell reporters what's going on and tell them when they get it wrong.

As he points out, however, the letter-to-the-editor page is no longer the primary locus of that interaction. It's just part of the "Long Tail" effect of the powerful new tools that he effectively documents, ones that are utterly changing the ways in which we do business and the ways in which we interact.

John Freeman, of New York City, is writing a book on the tyranny of e-mail for Scribner.