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 Chief Executive 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.

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.

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.

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."