Two members of the US Congress have asked Facebook to explain how applications transmitted information about users to advertising and Web tracking companies in violation of the social network's rules.

Representative Joe Barton, a Republican from Texas, and Representative Edward Markey, a Democrat from Massachusetts, asked Facebook founder Mark Zuckerberg to reply to 18 questions about what they called a "privacy breach."

Facebook on Monday acknowledged that some popular third-party applications had passed on user identification (UID) information but played down the privacy implications.

"Nevertheless, we are committed to ensuring that even the inadvertent passing of UIDs is prevented and all applications are in compliance with our policy," Facebook engineer Mike Vernal said in a blog post.

"Our policy is very clear about protecting user data, ensuring that no one can access private user information without explicit user consent," Vernal said. "We take strong measures to enforce this policy, including suspending and disabling applications that violate it."

In their letter, Burton and Markey, co-chairmen of the House BiPartisan Privacy Caucus, said: "Given the number of current users, the rate at which that number grows worldwide, and the age range of Facebook users, combined with the amount and the nature of information these users place in Facebook's trust, this series of breaches of consumer privacy is a cause for concern."

They asked how many users were impacted, what information was transmitted to other parties and how many third-party applications were involved.

They also asked whether Facebook will "seek the deletion of its users' personal information from data bases of the Internet or advertising companies who received it as a result of this series of privacy breaches?"

Andrew Noyes, a Facebook spokesman, said Facebook "is committed to safeguarding private data while letting people enjoy meaningful social experiences with their friends.

"As our privacy policy states, when a Facebook user connects with an application, the user ID is part of the information that the application receives," he said.

"The suggestion that the passing of a user ID to an application, as described in Facebook's privacy policy, constitutes a 'breach' is curious at best," Noyes said.

The Wall Street Journal said applications were providing access to Facebook members' names and, in some cases, their friends' names, to companies that build detailed databases on people in order to track them online.

All of the 10 most popular applications on Facebook were transmitting unique user ID numbers to outside companies, it said. They include Zynga's FarmVille, with 59 million users, Texas HoldEm Poker and FrontierVille.

Facebook is the world's most popular social network with around 500 million users, but it has been dogged by complaints about privacy protection.

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