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.

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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.”

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