The details of 100 million Facebook users - a fifth of the social networking site's members - have been posted online by a security analyst, in a stark demonstration of the potential privacy weaknesses of social networks.

In a detailed blog post, Ron Bowes of Skull Security explained that he used a simple piece of code to perform the scrape, which took any data not already locked down within personal privacy settings. However, as of this morning, his web site and the blog post were unavailable.

The list of users has been shared on peer-to-peer site The Pirate Bay, and included in the packaged files are names and Facebook URLs.

Facebook is calm about the hack, explaining that the information that was taken had already been made public by users.

"This information already exists in Google, Bing, other search engines, as well as on Facebook," the social network said.

"No private data is available or has been compromised. Similar to a phone book, this is the information available to enable people to find each other, which is the reason people join Facebook."

However, the firm is investigating whether the collection of information in this way was a violation of its terms and conditions.

Graham Cluley, senior technology consultant at security firm Sophos, concurred with Facebook's stance, explaining that it was enabled by lax user controls.

"This wasn't really a 'hack' as such, as the guy who collected this information didn't have to break into accounts to access the information," he said.