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.
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."
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 an option for suspending accounts (Look for a link under the "Security" tab in "Account Settings.")
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."
Associated Press Deputy Director of Polling Jennifer Agiesta and News Survey Specialist Dennis Junius in Washington contributed this report.