SAN JOSE, CALIF. -- Facebook may have 500 million friends, an Oscar-nominated film and an Egyptian revolution to its credit. But not everyone embraces the social network, including plenty of tech, sports and political luminaries.

That's right, there's a category of people who just might be too big -- or too busy -- to Facebook.

You can't "friend" Cisco's CEO John Chambers on Facebook, or Apple mastermind Steve Jobs (though you can find people pretending to be him). You won't find status updates from the Obama girls, or pithy posts from actor James Franco or hockey star Patrick Marleau.

"I don't use Facebook," Marleau, a winger for the San Jose Sharks, said after a practice last week. "The people who know me already know my number." And, he added, "You never know who is going to try to get hold of you, and the reasons behind it."

Indeed, high-profile people who shun Facebook have a variety of reasons for avoiding the planet's most popular social networking platform, from privacy concerns to time constraints.

"Facebook isn't for everyone," said Zizi Papacharissi, communications professor at the University of Illinois at Chicago and editor of the book "A Networked Self." "It's a technology, and you have to find a place for it in your life. Or maybe there isn't a place for it in your life."

Chief executives of publicly traded companies, for example, wouldn't want to talk business on Facebook, some experts say, for fear of posting something that might get them in trouble with stock market regulators, or even with shareholders.

But some CEOs have "fan" pages with updates and photos posted by their public relations teams. Chambers' page, for example, which recently featured news that Chambers will give Duke University's commencement address, states that "posts are not from John Chambers himself." Chambers doesn't use a personal Facebook account, says John Earnhardt, director of social media at Cisco.

"There's not a lot of time for him to go on and utilize it," Earnhardt said. Case in point: Chambers' schedule recently included being in Spain for a mobile conference and then back in the San Francisco Bay Area the next day for dinner with President Obama and some other tech bigwigs.

Concerns about privacy

Even though there are privacy controls on Facebook -- confusing and labyrinthine, some would say, but controls nonetheless -- many in high-visibility positions don't want to use the site for fear of spilling too much information about themselves in public. Last month, for example, First Lady Michelle Obama said during an interview with "The Today Show" that presidential daughters Malia and Sasha are not allowed to use Facebook.

And in a recent Newsweek interview with Oscar-nominated actors, when asked, none said they were on Facebook. Actor James Franco said he had been briefly, but quit because he didn't like the way his different groups of friends got slung together in one big pool.

Social media strategist Jeremiah Owyang of the Altimeter Group, based in San Mateo, Calif., said some celebrities are "concerned that social media will dilute their brand" by overexposing them.

Some celebs discover, just as the non-famous have, that Facebook doesn't suit them. Sharks center Joe Thornton was on Facebook for about a year, a few years back, and then dropped it. "There's no use for it for me," he said in a locker-room interview. "I like to keep in contact with about six people in my life, and that's about it."

Olympians and fake accounts

Of course, you can find luminaries who use Facebook for personal use, including Apple co-founder Steve Wozniak, Salesforce.com CEO Marc Benioff and Olympic ice skating champion Kristi Yamaguchi. Not to mention the numerous accounts that pretend to be those of famous people like Apple's Steve Jobs or Oracle's Larry Ellison. (Neither company would comment on the topic of high-profile Facebookers. Nor would Facebook.)

For some hyper-scheduled executives, Owyang said, Facebook can be a streamlined way to check in with one's personal circle.

Godfrey Sullivan, CEO of Splunk, a privately held San Francisco information technology company, said he enjoys using Facebook to keep up with friends and family only, no business contacts -- so much so that he'd even pay for the service.

If he gets "friend" requests from "characters whom you've known well from business, you like them, but you don't need them looking at your vacation pictures," he simply asks them to join his network on LinkedIn, the business-oriented online network.

"I tend to think if it as a digital family photo album," he said of Facebook. His two teenage daughters lavish their time on the site, he says, but he keeps his visits succinct, posting a few pictures and commenting on family updates.

He goes on Facebook once a week to read and once a month to post. "And then I'm off and back to work."