Facebook.com, the No. 2 U.S. social network site that is quickly expanding beyond its college-student base, has been met with a sudden privacy backlash by users after it made design changes this week.

By late on Wednesday, more than 500,000 of Facebook's 9.5 million members had signed an online petition calling for the company to back off a feature called "News Feed" that instantly notifies members when friends update their own sites.

On Monday, the Palo Alto, Calif.-based company, founded in 2004 by then-Harvard University student Mark Zuckerberg, began automatically notifying users whenever new photos were posted by friends. The site also makes it easier to learn about people's political or social affiliations or dating status.

• Click here to read about how potential employers may be combing through Facebook.com pages to determine hiring decisions.

Facebook differs from its bigger rival MySpace.com by giving members far greater control over every detail they may publish on their individual member profile pages.

Manifesting in record cyberspace time and scale, the protests have swept across college campuses, taking on the form of proto-political actions which are only possible in the age of instant electronic links and virtual mob behavior.

"Calm. down. breathe. We hear you," Zuckerberg wrote in a note to users on the Facebook site on Tuesday of the changes.

"We think they are great products, but we know that many of you are not immediate fans, and have found them overwhelming and cluttered," he wrote. "Other people are concerned that non-friends can see too much about them."

FRIENDS WHO KNOW TOO MUCH

The sudden reaction follows a raft of U.S. privacy scares sparked over the past year by the theft of key details on millions of credit-card users and by widespread reports of adult predators targeting teenage users of MySpace.

By contrast, the Facebook reaction is fueled not because it revealed any new personal data about its users. Rather, the change simply makes it easier for friends to track one another.

"Stalking is supposed to be hard," a Facebook user complained.

"News Feed is just too creepy, too stalker-esque, and a feature that has to go," reads the petition of the newly formed "Students against Facebook News Feed."

Nonetheless, the outrage mingled with tongue-in-cheek humor as evidenced in the name of a related protest site: "The Coalition to Stop Facebook, Stalker Edition."

Both groups can be reached only by registered Facebook members at http://www.facebook.com/group.php?gid=2208288769.

Zuckerberg responded in his Tuesday blog post to members that, "We agree, stalking isn't cool; but being able to know what's going on in your friends' lives is."

"Nothing you do is being broadcast; rather, it is being shared with people who care about what you do — your friends."

One of the many impromptu protest sites that have formed called for users to boycott Facebook in a day of protest on September 12. That site is at http://daywithoutfacebook.blogspot.com/.

The threat of a boycott comes at a vulnerable time for Facebook, which was bracing for a surge in subscribers as students return to school this month at the roughly 2,100 colleges and 22,000 high schools that offer Facebook accounts.

Facebook has roughly one-tenth the number of users of rival social networking site MySpace.com, which accepts members of all ages and has become a big hit among high-school age students. MySpace is a unit of News Corp. (NWS)

Privacy has been a selling point for Facebook, which took a more disciplined approach to growth by restricting membership initially only to Internet users with ".edu" e-mail addresses — which require affiliation with an educational institution.

Four months ago, it began offering accounts to select organizations, including major companies, non-profit groups and members of the military. So far, 15,000 organizations have been added to the list of Web addressees that qualify for Facebook.

"What we are trying to do is get people to share information — and to share whatever they are comfortable doing," Zuckerberg told Reuters regarding the changes.

FOXNews.com and MySpace.com are both owned and operated by News Corporation.