More than 900 million people worldwide check their Facebook accounts at least once a month, but millions more are Facebook holdouts.

They say they don’t want Facebook. They insist they don’t need Facebook. They say they’re living life just fine without the long-forgotten acquaintances that the world’s largest social network sometimes resurrects.

They are the resisters.

“I’m absolutely in touch with everyone in my life that I want to be in touch with,” MaLi Arwood says. “I don’t need to share triviality with someone that I might have known for six months 12 years ago.”

Even without people like Arwood, Facebook is one of the biggest business success stories in history. The site had 1 million users by the end of 2004, the year Mark Zuckerberg started it in his Harvard dorm room. Two years later, it had 12 million. Facebook had 500 million by summer 2010 and 901 million as of March 31, according to the company.

We are Facebook, you will be assimilated

That staggering rise in popularity is one reason why Facebook Inc.’s initial public offering is one of the most hotly anticipated in years. The company’s shares are expected to begin trading on the Nasdaq Stock Market on Friday under the ticker symbol “FB”. Facebook is likely to have an estimated market valuation of some $100 billion, making it worth more than Kraft Foods, Ford or Disney.

Facebook still has plenty of room to grow, particularly in developing countries where people are only starting to get Internet access. As it is, about 80 percent of its users are outside U.S. and Canada.

But if Facebook is to live up to its pre-IPO hype and reward the investors who are clamoring for its stock this week, it needs to convince some of the resisters to join. Two out of every five American adults have not joined Facebook, according to a recent Associated Press-CNBC poll. Among those who are not on Facebook, a third cited a lack of interest or need.

If all those people continue to shun Facebook, the social network could become akin to a postal system that only delivers mail to houses on one side of the street. The system isn’t as useful, and people aren’t apt to spend as much time with it. That means fewer opportunities for Facebook to sell ads.

Lee Rainie, director of the Pew Internet & American Life Project, says that new communications channels – from the telephone to radio, TV and personal computers – often breed a cadre of holdouts in their early days.

“It’s disorienting because people have different relationships with others depending on the media they use,” Rainie says. “But we’ve been through this before. As each new communications media comes to prominence, there is a period of adoption.”

Fine for grandkids

Len Kleinrock, 77, says Facebook is fine for his grandchildren, but it’s not for him.

“I do not want more distractions,” he says. “As it is, I am deluged with email. My friends and colleagues have ready access to me and I don’t really want another service that I would feel obliged to check into on a frequent basis.”

Kleinrock says his resistance is generational, but discomfort with technology isn’t a factor.

After all, Kleinrock is arguably the world’s first Internet user. The University of California, Los Angeles professor was part of the team that invented the Internet. His lab was where researchers gathered in 1969 to send test data between two bulky computers -the beginnings of the Arpanet network, which morphed into the Internet we know today.

“I’m having a ‘been-there, done-that’ feeling,” Kleinrock says. “There’s not a need on my part for reaching out and finding new social groups to interact with. I have trouble keeping up with those I’m involved with now.”

Thomas Chin, 35, who works at an advertising and media planning company in New York, says he may be missing out on what friends-of-friends-of-friends are doing, but he doesn’t need Facebook to connect with family and closer acquaintances.

“If we’re going to go out to do stuff, we organize it (outside) of Facebook,” he says.

Some people don’t join the social network because they don’t have a computer or Internet access, are concerned about privacy, or generally dislike Facebook. Those without a college education are less likely to be on Facebook, as are those with lower incomes. Women who choose to skip Facebook are more likely than men to cite privacy issues, while seniors are more likely than those 50-64 years old to cite computer issues, according the AP-CNBC poll.

Half of under 35s use it

About three-quarters of seniors are not on Facebook. By contrast, more than half of those under 35 use it every day.

The poll of 1,004 adults nationwide was conducted by GfK Roper Public Affairs and Corporate Communications May 3-7 and has a margin of sampling error of plus or minus 3.9 percentage points.

Steve Jones, a professor who studies online culture and communications at the University of Illinois at Chicago, says many resisters consider Facebook to be too much of a chore.

“We’ve added social networking to our lives. We haven’t added any hours to our days,” Jones says. “The decision to be online on Facebook is simultaneously a decision not to be doing something else.”

Jones says many people on Facebook try to overcome that by multitasking, but they end up splitting their attention and engaging with others online only superficially.

Arwood, 47, a restaurant manager in Chicago, says she was surprised when colleagues on an English-teaching program in rural Spain in 2010 opted to spend their breaks checking Facebook.

“I spent my time on break trying to learn more about the Spanish culture, really taking advantage of it,” she says. “I went on walks with some of the students and asked them questions.”

Privacy concerns

Kariann Goldschmitt, 32, a music professor at New College of Florida in Sarasota, Fla., was on Facebook not long after its founding in 2004, but she quit in 2010. In part, it was because of growing concerns about her privacy and Facebook’s ongoing encouragement of people to share more about themselves with the company, with marketers and with the world.

She says she’s been much more productive since leaving.

“I was a typical user, on it once or twice a day,” she says. “After a certain point, I sort of resented how it felt like an obligation rather than fun.”

Besides Facebook resisters and quitters, there are those who take a break. In some cases, people quit temporarily as they apply for new jobs, so that potential employers won’t stumble on photos of their wild nights out drinking. Although Facebook doesn’t make it easy to find, it offers options for both deleting and suspending accounts.

Goldschmitt says it takes effort to stay in touch with friends and relatives without Facebook. For instance, she has to make mental notes of when her friends are expecting babies, knowing that they have become so used to Facebook “that they don’t engage with us anymore.”

“I’m like, ‘Hmmm, when is nine months?’ I have to remember to contact them since they won’t remember to tell me when the baby’s born.”

Neil Robinson, 54, a government lawyer in Washington, says that when his nephew’s son was born, pictures went up on Facebook almost immediately. As a Facebook holdout, he had to wait for someone to email photos.

After years of resisting, Robinson plans to join next month, mostly because he doesn’t want to lose touch with younger relatives who choose Facebook as their primary means of communication.

But for every Robinson, there is an Edelstein, who has no desire for Facebook and prefers email and postcards.

“I prefer to keep my communications personal and targeted,” says Jake Edelstein, 41, a pharmaceutical consultant in New York. “You’re getting a message that’s written for you. Clearly someone took the time to sit down to do it.”