Facebook claims that in just six years it has reached the 500-million member mark. If Facebook were a country, it would be the world's third largest in population, behind China and India.

The mobile segment is the fastest growing, with the number of mobile users increasing 50% in the past three months, to about 150 million.

Facebook chief executive Mark Zuckerberg put the number of mobile users at 100 million in April, which suggests a 50% growth in the past three months.

"This is an important milestone for all of you who have helped spread Facebook around the world," said Zuckerberg in a blog post.

"Now a lot more people have the opportunity to stay connected with the people they care about," he said.

The milestone proves that at least 8% of the world's population is more concerned about being 'connected' than about their privacy.

In May, Facebook added security tools and simplified its privacy settings, but many rights groups have said the moves do not go far enough to protect members.

As Zuckerberg celebrates the phenomenal growth of Facebook into a business worth around $22bn, he is facing a website developer's claim to own 84% of the company.

New Yorker Paul Ceglia claims he signed a contract with Zuckerberg in 2003 to develop a website that entitled him to a 50% stake in the product that eventually became Facebook.

He also claims the contract awarded him an additional 1% stake for every day of the project, giving him a total stake of 84%.

Facebook has described the lawsuit as "frivolous" and Zuckerberg has said he is "quite sure" he did not sign such a contract, according to Bloomberg.com.

Zuckerberg made the statement in an interview after Facebook lawyer Lisa Simpson told a judge the company was "unsure" on the matter.

This story was first published by Computer Weekly