Facebook to welcome its 500 millionth user today

Facebook, the social networking phenomenon started in a Harvard dorm six years ago, could have 500 million members as early as today.

Mark Zuckerberg, the group's 26-year-old chief executive, is expected to announce the news in an interview with Diane Sawyer on ABC World News in the US this evening.

Insiders said the site was likely to hit its latest milestone, which takes its reach to about 8 per cent of the world's population, just five months after it said it had 400 million members. In January last year, Facebook had 150 million users.

However, a source close to the company cautioned that while it expected to hit the target this week, it "cannot confirm exactly when this will happen".

The announcement has been widely anticipated, especially with the online release of trailers for the forthcoming movie The Social Network, which chronicles the birth of Facebook and has the tagline: "You don't get to 500 million friends without making a few enemies".

Last month, Mr Zuckerberg told the Cannes Lions International Advertising Festival that there was a "good chance" of bringing Facebook to one billion people within the next three to five years. The network is heavily targeting international expansion and has moved to tailor the website to different countries. There are only four markets in which Facebook is not the dominant player: Japan, South Korea, China and Russia.

The chief executive is expected to face intense questioning from Sawyer about a series of issues, including his company's attitude to privacy. Earlier this year, Facebook was criticised for controversial changes to its privacy policy and was forced to respond.

Last month, Mr Zuckerberg met David Cameron at Downing Street to offer the Prime Minister advice about communicating in the era of social networking. Facebook has an estimated 26 million members in the UK.