Facebook, the fast-growing social network, has been forced to update its privacy settings after it was pointed out that users may unwillingly have been exposing certain personal details on the site.

Privacy officers at Facebook were alerted to the problem by a blog post which claimed that, even if users had set their profile to 'private', certain information - such as their sexual preference and religious beliefs - could be revealed in a search of the site.

The situation meant that anyone could search for a user's name and, by narrowing the query according to various categories, find out details that the user did not intend to disclose.

Defenders of the site said that users had always been able to limit what people browsing could find out about them using the search function, but Facebook administrators said that it had been a mistake not to have the privacy settings in search accord by default with those in a user's profile.

"If you undertook an advanced search, it may have been possible to see a piece of information in a person's profile, even if that profile was private," Chris Kelly, chief privacy officer at Facebook told Times Online.

"Information that was available through search did not adequartely match the information in a profile, but we've now fixed that," he said, adding that the site's "mission" remained to leave control over information "in the hands of users."

In a blog post that was picked up widely on the internet, Chris Soghoian wrote: "A large number of (Facebook) users have gone to the effort to restrict who can view their profiles, but many, unfortunately, remain exposed to a trivial attack."

The post was picked up on Wired.com, and soon after, Facebook representatives replied on Mr Soghoian's blog that the site had updated its settings "so that profile information that has been made private by a user, such as gender, religion, and sexual orientation, will not return a result."

Anna Martinez, 29, who has used Facebook since November, said: "The whole reason Facebook is so appealing to people who don't normally go online to socialise is the selectivity about who gets to see your stuff and who doesn't," Anna Martinez, 29, who has used Facebook since November, said.

"I set my privacy setting so that only my "friends" could see me. To discover that people might have been able to find out information through this loophole makes me feel vulnerable, and a little scared."

A spokesman for the Information Commissioner's Office said: "We would continue to warn people that there are risks associated with posting information online, and urge them to give out personal information with great care."

Facebook had 3.2 million visitors in the UK in May, making it the third most popular network behind MySpace and Bebo.

Its audience has increased by more than 500 per cent in the past 6 months - nearly 20 times the growth rate of MySpace's.

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