It's as ubiquitous as aspirin, and if you believe its adherents, just as important.

Just six years after its founding, social-networking site Facebook will lure its 500 millionth user this week, cementing its status as the most popular site in the history of the internet and - along with Google - probably the most influential.

The company started as a programming pet project for Harvard undergraduate Mark Zuckerberg, who modeled the site on the "facebooks" used by schools and colleges to enable students to get to know their classmates.

His first project landed him in hot water with authorities at his Ivy League university after he hacked into the college website to get pictures of students. But they wisely dropped charges against Zuckerberg when it became clear that the young man was on course to change the world.

Launched in February 2004, Facebook was initially limited to Harvard students. Then it expanded to Stanford, Columbia and Yale, then to other Boston colleges and gradually to colleges all over the US. The next step was to open it to all high school students in September 2005 and a year later anyone age 13 and up with a valid email address could register.

The site's explosion has been the ultimate exposition of viral growth.

It took more than three years to amass its first 100 million users and just 225 days to attract its second 100 million users. Facebook passed the 300 million mark in another 160 days and achieved its fastest rate of growth when it reached 400 million users in just another 143 days, when it was registering almost 700,000 new members daily.

Now the rate seems to be slowing a little - as it took 170 days to add enough users to reach the 500 million mark.

Like a teenager struggling to adapt to his changing body, Facebook has grappled with its meteoric rise as the online vault of our relationships, surfing habits, photographs, thoughts, experiences and every other facet of both our virtual and real-world lives.

"A big part of the challenge that we've had is that we've grown from tens of thousands of users to hundreds of millions," Zuckerberg said at a news conference he called in May following a storm of protest about the site's privacy policy changes.

"It's been a big shift along the way, and it hasn't always been smooth."

As people spend more and more time online, and spend more and more of their online time on Facebook, Zuckerberg has become a poster boy for the young tech billionaire set. But he claims not to care about mmoney and has consistently refused to yield control of the site to the money men. He is reluctant to cash in his stake with a multi-billion dollar initial public offering, which according to some estimates would value the company at more than 20 billion dollars.

Even Zuckerberg's recent privacy missteps were motivated not by a a rapacious hunt for lucrative data, but rather to further his vision of radical transparency, according to David Kirkpatrick, whose book

The Facebook Effect is one of the most perceptive examinations to date of the site.

According to Kirkpatrick, Zuckerberg sees Facebook as a social movement dedicated to the idea of radical transparency. He believes that by sharing our data and our public lives we become better people, less able to manipulate, connive and indulge in hypocrisy.

Some may see these ideals as noble, others as downright creepy. But most users will ignore the ideological implications as long as the site remains useful - or, more specifically, as long as their friends keep using the addictive site.

Facebook's hegemony faces other threats. Just as one can become sick of eating too many cream cakes, there's also a growing sense of Facebook overload, when the endless updates, pokes and suggestions from hundreds of "friends" all become too much to bear, says internet analyst Carmi Levy.

"Facebook is very clearly no longer a fad but its own landscape and a major force on the global internet," Levy says.

The company declared it was making a profit on advertising last year and "has nowhere to go but up" in terms of making money, Levy believes.

Levy predicts a drop in time that users spend on the site but has little idea if any company will ever be strong enough to lure away Facebook's massive user base.

"Something may replace Facebook one day," he says. "But if it does, it will be something we've never heard of."