Tyson Balcomb quit Facebook after a chance encounter on an elevator. He found himself standing next to a woman he had never met -- yet through Facebook he knew what her older brother looked like, that she was from a tiny island off the coast of Washington and that she had recently visited the Space Needle in Seattle.
"I knew all these things about her, but I'd never even talked to her," said Mr. Balcomb, a pre-med student in Oregon who had some real-life friends in common with the woman. "At that point I thought, maybe this is a little unhealthy."
As Facebook prepares for a much-anticipated public offering, the company is eager to show off its momentum by building on its huge membership: more than 800 million active users around the world, Facebook says, and roughly 200 million in the United States, or two-thirds of the population.
But the company is running into a roadblock in this country. Some people, even on the younger end of the age spectrum, just refuse to participate, including people who have given it a try.
One of Facebook's main selling points is that it builds closer ties among friends and colleagues. But some who steer clear of the site say it can have the opposite effect of making them feel more, not less, alienated.
"I wasn't calling my friends anymore," said Ashleigh Elser, 24, who is in graduate school in Charlottesville, Va. "I was just seeing their pictures and updates and felt like that was really connecting to them."
To be sure, the Facebook-free life has its disadvantages in an era when people announce all kinds of major life milestones on the Web. Ms. Elser has missed engagements and pictures of newborn babies. But none of that hurt as much as the gap she said her Facebook account had created between her and her closest friends. So she shut it down.
Many of the holdouts mention concerns about privacy. Those who study social networking say this issue boils down to trust. Amanda Lenhart, who directs research on teenagers, children and families at the Pew Internet and American Life Project, said that people who use Facebook tend to have "a general sense of trust in others and trust in institutions." She added: "Some people make the decision not to use it because they are afraid of what might happen."
Ms. Lenhart noted that about 16 percent of Americans don't have cellphones. "There will always be holdouts," she said.
Facebook executives say they don't expect everyone in the country to sign up. Instead they are working on ways to keep current users on the site longer, which gives the company more chances to show them ads. And the company's biggest growth is now in places like Asia and Latin America, where there might actually be people who have not yet heard of Facebook.
"Our goal is to offer people a meaningful, fun and free way to connect with their friends, and we hope that's appealing to a broad audience," said Jonathan Thaw, a Facebook spokesman.
But the figures on growth in this country are stark. The number of Americans who visited Facebook grew 10 percent in the year that ended in October -- down from 56 percent growth over the previous year, according to comScore, which tracks Internet traffic.
Ray Valdes, an analyst at Gartner, said this slowdown was not a make-or-break issue ahead of the company's public offering, which could come in the spring. What does matter, he said, is Facebook's ability to keep its millions of current users entertained and coming back.
"They're likely more worried about the novelty factor wearing off," Mr. Valdes said. "That's a continual problem that they're solving, and there are no permanent solutions."
Erika Gable, 29, who lives in Brooklyn and does public relations for restaurants, never understood the appeal of Facebook in the first place. She says the daily chatter that flows through the site -- updates about bad hair days and pictures from dinner -- is virtual clutter she doesn't need in her life.
"If I want to see my fifth cousin's second baby, I'll call them," she said with a laugh.
Ms. Gable is not a Luddite. She has an iPhone and sometimes uses Twitter. But when it comes to creating a profile on the world's biggest social network, her tolerance reaches its limits.
"I remember having MySpace for a bit and always feeling so weird about seeing other people's stuff all the time," she said. "I'm not into it."
Will Brennan, a 26-year-old Brooklyn resident, said he had "heard too many horror stories" about the privacy pitfalls of Facebook. But he said friends are not always sympathetic to his anti-social-media stance.
"I get asked to sign up at least twice a month," Mr. Brennan said. "I get harangued for ruining their plans by not being on Facebook."
And whether there is haranguing involved or not, the rebels say their no-Facebook status tends to be a hot topic of conversation -- much as a decision not to own a television might have been in an earlier media era.
"People always raise an eyebrow," said Chris Munns, 29, who works as a systems administrator in New York. "But my life has gone on just fine without it. I'm not a shut-in. I have friends and quite an enjoyable life in Manhattan, so I can't say it makes me feel like I'm missing out on life at all."
But the peer pressure is only going to increase. Susan Etlinger, an analyst at the Altimeter Group, said society was adopting new behaviors and expectations in response to the near-ubiquity of Facebook and other social networks.
"People may start to ask the question that, if you aren't on social channels, why not? Are you hiding something?" she said. "The norms are shifting."
This kind of thinking cuts both ways for the Facebook holdouts. Mr. Munns said his dating life had benefited from his lack of an online dossier: "They haven't had a chance to dig up your entire life on Facebook before you meet."
But Ms. Gable said such background checks were the one thing she needed Facebook for.
"If I have a crush on a guy, I'll make my friends look him up for me," Ms. Gable said. "But that's as far as it goes."