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.