Facebook on Friday took more steps to stop third-party applications from sharing identifying information about users with advertising and Internet tracking companies.

"Today, we are clarifying our policy to ensure that developers understand the proper use of UIDs (user identification data) in their applications," the world's leading online social network said in a release.

"Our policy has always stated that data received from Facebook, including UIDs, cannot be shared with data brokers and ad networks."

Facebook recently learned that some software developers behind outside applications popular in the online community were "inadvertently" sharing user identification numbers due to the way Web browser programs work.

"We have proposed a technical solution to prevent this sort of transfer in the future," Facebook said. "In addition, we are working with browser vendors to address this issue more broadly across the web."

Facebook modified its policy to forbid user identification data from leaving third-party applications and for such information to remain confidential if used by analytics services connected to programs.

"Ad networks that operate on Facebook Platform are already required to sign terms that govern their use of data," the social network said.

"We are requiring these ad networks to delete any Facebook UIDs, regardless of how they were obtained.

Facebook said that its investigation into the situation revealed that some developers were paid by a data broker for UIDs in violation of the social network's policy.

Violating developers are being placed on 6-month moratoriums and will have their data practices audited to make sure they are in compliance with rules at the social network, according to Facebook.

The moratorium was said to affect fewer than a dozen, mostly small developers, none of which are in the top ten applications at Facebook.

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."

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.