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.