"The basic notion that people reach a technological saturation point applies to a lot of people," says Lee Rainie, the director of the Pew Research Center's Internet and American Life Project, which studies the Web's impact on society. "They literally say, enough is enough . . . and my mind is going to blow up and I can't take it anymore."

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One problem, Rainie says, is that so many of the so-called revolutionary applications are actually riffs on a similar theme. Foursquare looks a lot like Gowalla; Tumblr could be the cousin of Posterous. It's exhausting to move onto what's new if it sort of looks like what's old. The people who still refuse to Facebook -- the most ubiquitous of them all, with upwards of 500 million members and a Quantcast-estimated 135 million daily visitors -- use this rationale: They already joined Friendster in 2004. Is that not enough?

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