Apple's iPhone may be big news, but in Silicon Valley all eyes are on Facebook .

The all-ages private Internet club that began as a college student hangout has become the tech industry's hottest ticket.

Facebook's membership among the early adopters who are first to latch onto new technology trends has skyrocketed in recent weeks, sparking a minor backlash among kids who find it "creepy" that adults are flooding to their site.

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The Palo Alto-based company, founded in 2004 as a social site for students at Harvard by then-undergraduate Mark Zuckerberg , has seen usage spike by allowing hundreds of software makers to build programs for the site.

Indeed, Venture capitalists are now quizzing Web entrepreneurs on their "Facebook strategy."

"Call it escape velocity or whatever you want, but social networking now has it. Facebook now has it," Netscape co-founder Marc Andreessen wrote last week on his blog.

His latest company, Ning, is helping users build simple software programs that can easily be made to run within Facebook.

"Do not miss what's happening to Facebook. It is turning mainstream," notes popular technology blogger Robert Scoble.

Facebook (http://www.facebook.com ), which started out offering simple Web profile pages for users to update personal details and link to their friends, has since late May provided an easy way to enhance profiles with a growing menu of 1,400 applications from virtual horoscopes to music video players.

Membership has exploded to more than 29 million active users, up one million users in just the past week and 5 million from six weeks ago. It's adding more than 150,000 members a day, up from its pace of 100,000 six weeks ago, Facebook says.

VIRTUAL SOCIETY AND ITS DISCONTENTS

Facebook has become the central way many users keep track of Web sites they use every day. More importantly, it's a way to keep up with what friends are doing, says Chief Technology Officer Adam D'Angelo, a high school classmate of Zuckerberg.

Far bigger rival MySpace has difficulty striking a balance between sharing personal data and not divulging "too much information." Many Facebook users post their mobile phone numbers, political affiliations or changes in dating status.

"Facebook is inherently not open the way the Web is open. Users share all kinds of information on the site they would never share on the Web," D'Angelo, 22, says. "We get users to divulge more information because we protect users' privacy.

Facebook's core audience of high-school and college students is uncertain what to make of the arrival of business users, teachers and authority figures of all sorts, even parents.

One protest group, called "Facebook Should Be Students ONLY" with 30,000 members, is a magnet for random discontent.

"Facebook should mos def (most definitely) be ONLY students," Katerina Laurel, 15, of Kansas City, Kansas, writes in Web shorthand. "Our CHOIR teacher has (a Facebook membership)."

Laurel's solution is to use the site's privacy controls to exclude the teachers/principals/deans/choir directors in her life.

"i can just block them from my site if i dont want them to see any of my profile," she says.

SUDDEN CONVERTS

CEO Zuckerberg, 23, unveiled the applications platform at a May 24 event in San Francisco by declaring: "Today, together, we are going to start a movement."

Several bloggers sniped at his immodesty, and a reporter from a major newspaper complained about having to cover the "Facebook hype."

Yet within days of the announcement, Facebook was having to contend with becoming the victim of its own success. Some programmers complained it was hard to get noticed in the flood of programs debuting each day.

Slide Inc. tops the list of application creators with a variety of programs including Top Friends, which puts up photos of a user's best friends on Facebook. It has 7.6 million users.

ILike is the second most popular with 4.3 million users of software that allows friends to listen to your favorite songs.

Freelance developer Craig Ulliott of Philadelphia captured the feeling when he asked on a forum for software programmers within Facebook: "I have 250,000 users, what now?"

His travel map application "Where I've Been" was adding several users every second, overwhelming his ability to pay for computers to support the increase in traffic. Since raising the issue, he's added nearly a million more users to 1.2 million in all.

Critics have seized on his conundrum, noting that Facebook makes its developers bear the cost of serving users but, so far, gives them few ways to make money.

Venture capitalist Brad Feld wrote on his blog: "As far as I can tell, none of these Facebook developers are deriving any real benefits.

The overnight popularity of many software programs also has led detractors to argue that many of the applications gaining traction on Facebook are unsophisticated and superficial programs -- emoticons, horoscopes, even virtual food fights.

To sort through the avalanche of new programs, Rodney Rumford, a blogger and corporate business consultant, has launched FaceReviews, a software evaluation site.

He praises programs that tap into the social connections which bring together members and condemns products that take users off of the Facebook site.

Rumford, 44, of Solana Beach, near San Diego, says Yahoo's week-old music video application marks the arrival of more sophisticated programs. It gives Facebook users an entertaining way to define their music tastes and share them with friends.

Facebook's D'Angelo chides critics, saying that the rules of building advanced software have not been overturned.

"Now that the platform has been out for six weeks, we are finally seeing applications that take a month to develop," he said.