Social network giant Facebook has registered its 500 millionth member, the firm has announced.
The site, which launched in 2004, has gained around 100 million new users in the last six months.Facebook said the number was "an important milestone" and added that it was "humbled and inspired" by the stories of its users, which it is asking people to share on the site.
The UK currently has around 26m Facebook users.
Stuart Miles of tech blog Pocket Lint said its success was down to "word of mouth".
"There are so many people online and so many people wanting to share with each other's friends - it's like a snowball," he told BBC News.
"It started off small, rolled and rolled, then all of a sudden not just your immediate circle of friends are on it but loads of people are, and they're all evangelising about why you should be on it too.
"The next milestone, which will probably be 750m, won't take as long - the more you have the more it spreads, like that snowball."
Mr Zuckerberg recently said that the site was "almost guaranteed" to reach one billion users.
But influential blogger Robert Scoble told the BBC that, despite the controversy, most people were not put off.
"There is no expectation of privacy there," he said. "Privacy stuff does not resonate with 'normal' people."
As Facebook has grown, many of its competitors have shrunk or disappeared.
MySpace, owned by News Corp, was once the poster child of social networking.
Facebook's director of European policy, Richard Allan, talks about the site's future aims
"MySpace is aimed at kids and they have limited access to computers," said Mr Miles.
Another competitor, Bebo, has also faced difficulty. In June this year its owner, AOL, sold the site just two years after it bought it for $850m (£417m).
The company said it was unable to provide the "significant investment" Bebo needed to compete with its social networking rivals.
One site that appears to be growing alongside Facebook is Twitter.
Measurements of Twitter traffic are notoriously difficult as many users do not interact with the service through the website. Instead, they use desktop software and mobile phones. However, it is estimated to be approaching 200m users, including Mr Scoble.
"I love [Facebook] for talking to personal friends, keeping up to date with what they're doing," he said.
"But I'm more of a Twitter user, I use that to get information out and talk to a community that cares about the same things that I do."