"As of this morning, 500 million people all around the world are actively using Facebook to stay connected with their friends and the people around them," Facebook founder Mark Zuckerberg said in a blog post.

"This is an important milestone for all of you who have helped spread Facebook around the world."

To celebrate, the California firm introduced an application that lets members of the online community "tell the incredible stories of the moving and interesting ways they've used Facebook."

Examples given by Zuckerberg included NATO Secretary General Anders Fogh Rasmussen jogging with Facebook fans during his term as Danish prime minister and a US woman using the service to battle breast cancer.

"Our mission at Facebook is to help make the world more open and connected," Zuckerberg said.

"I could have never imagined all of the ways people would use Facebook when we were getting started 6 years ago."

Zuckerberg has pointed to Facebook's unrelenting growth to rebuff criticism of feature changes or privacy safeguards at the website.

Americans are increasingly obsessed with Facebook and many young women check their page even before using the bathroom in the morning, according to a poll released last week.

However, a US study released on Tuesday indicates that while people may be addicted to Facebook they rank it near the bottom when it comes to customer satisfaction.

Facebook landed with notoriously despised airlines and cable television companies in the bottom 5 percent of private companies ranked in a 2010 American Customer Satisfaction Index E-Business Report produced in partnership with ForeSee Results.

"Our research shows that privacy concerns, frequent changes to the website, and commercialization and advertising adversely affect the consumer experience," ForeSee chief executive Larry Freed said in a release.

Zuckerberg openly advocates Internet firms releasing innovations quickly and "iterating" with improvements.

"It's clear that while innovation is critical, sometimes consumers prefer evolution to revolution," Freed said.

Facebook recently overhauled privacy controls in the face of a barrage of criticism that it is betraying the trust which has made it the world's biggest social network.

Facebook is growing despite criticisms because "there really isn't a strong alternative and Facebook is relatively sticky, migrating off is a ton of work," said analyst Rob Enderle of Enderle Group in Silicon Valley.

Leaving Facebook ostensibly involves finding a new online home for photos and other digital content from profile pages and then convincing friends to join one there.

"People keep joining Facebook because that is where their friends are," Enderle said.

"Until there is a strong alternative, we won't even have the beginnings for change. And the more people that join Facebook, the stronger that competitor is going to have to be."

Technology titans Google and Microsoft have the resources and the motivation to field or back a Facebook rival, according to the analyst.

Microsoft bought a small stake in Facebook three years ago for 240 million dollars (US).

Microsoft added Facebook to Outlook last week, giving users of its popular email program the ability to view status updates, pictures and wall posts from their friends on the social network.

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