A glitch in a test version of Facebook's Web site inadvertently exposed the birthdays of Facebook's 80 million members this week.

The bug was discovered over the weekend by Sophos Senior Technology Consultant Graham Cluley. While checking out Facebook's new design, Cluley noticed that the birth dates of some of his privacy-obsessed acquaintances were popping up when they should have been hidden.

Facebook allows users to control who sees private information such as their birth date, which can be a valuable nugget of data for identity thieves. But Cluley discovered that the new site was making this information public to other members. "Their new profile page essentially ignored the privacy setting to withhold the data of birth," he said.

"For a brief period of time, a small number of users were able to access a private beta of Facebook's new site design meant only for developers. During that time, some of those users had their birthdays revealed due to a bug," Facebook said Wednesday in a statement. The company could not say exactly how long this data was exposed or how many people viewed the beta site, but the bug was patched within hours of Cluley's discovery.

Facebook may intend for the beta site to be private, but it has been open to the general public for several days. It features a new profile design that should be rolled out as an option to Facebook users some time this week.

Cluley himself did not consider this a major data breach, but he said it should serve as a warning to people who put a lot of information on social networks. "It raises a more serious question which is, 'Can you trust these social networks to look after your data properly?'" he said.

Facebook is sensitive about privacy. In November the company scrambled to fix its Beacon ad system after a CA researcher discovered that the system was collecting data on users' online behavior, despite Facebook's assurances to the contrary.

"With Beacon we just screwed it up," said Matt Cohler, the company's vice president of product management, during a March session with reporters.

Cluley isn't sure that won't happen again. He's telling his friends to just make up a birth date on Facebook from now on.