FACEBOOK HAS signed up its 500-millionth user, almost four years after the site became widely available to the public.

It is now not only the world’s biggest social network, but also the fastest growing. Its co-founder and chief executive, Mark Zuckerberg (26), now thinks a target of a billion users is “almost guaranteed” given that it is the biggest social network in every country except Russia, Japan and China.

The site began in February 2004 with a college project for people who had gone to Harvard University. It quickly grew to other universities, and then secondary schools, until finally in September 2006 the site threw open its doors to anyone over 13.

Facebook’s own user figures are based on members that have used the site within the past month: of those 500 million, it says, half use the site every day, for an average of 34 minutes. About 150 million users access the site by mobile.

“This is the quiet revolution for Facebook,” says the company’s head of policy for Europe, Richard Allan. “We are well positioned for the world beyond the broadband-connected PC.”

But even as it zooms past that astonishing metric – having in the past six years been seen as key in the election of the US president as well as feted by the British prime minister as a means of connecting with people – it also faces challenges.

Those who have studied the growth of social media warn that its moment in the sun could soon pass – just as it has for one-time social network giants such as Friends Reunited, Bebo and MySpace.

Zuckerberg has had to weather lawsuits from people who claim to have built the site with him. In 2008, Facebook paid $65 million to end claims that he stole the idea. Another case, from a web designer who claims 84 per cent ownership of the site, awaits a hearing in a US court.

Zuckerberg has also been forced to defend the site’s ever-changing approach to users’ privacy, remodelling the way personal data is exposed to the rest of the internet.

Through it all, however, the site has kept on growing relentlessly. It even boasts its own virtual currency – Facebook credits – used by games and apps for payment between users. And it is profitable, through sales of ads and cuts on transactions made using its currency – having reportedly rebuffed billion-dollar offers from giants such as Google and Yahoo. Gartner analyst Monica Basso says the scale of the site now confirms it as “the mother of all social networks”. – (Guardian service)