Facebook has changed its three-week-old advertising platform to soothe members outraged by what they saw as an assault on their privacy at the popular social networking website. Facebook said Friday that members will only be fodder for the ad platform, referred to as Beacon, if they "opt-in" as opposed to the original format that automatically included them unless they took the effort to "opt-out."

Beacon lets "partners" track Facebook members' visits to their websites and relay messages letting users' friends in the social networking community know what they bought in a tactic referred to as "trusted referral" advertising.

Internet civic and political action group Moveon.org said that 55,000 of Facebook's 50 million members have electronically signed a petition titled "Facebook: Stop invading my privacy."

The petition calls for Facebook not to spread word of what members buy to their friends without explicit permission.

"We appreciate feedback from all Facebook users and made some changes to Beacon," the Northern California-based firm said in a written release.

Facebook members now get a notification asking them to click on an "OK" icon if they want stories about their activities at advertisers' websites to be sent to friends via automated news feeds.

If members do nothing with the notices, no stories are sent, according to Facebook, which acts as intermediary between advertisers and members. Facebook is adamant it does not share users' data with advertisers.

Facebook launched Beacon in early November in a move awaited by analysts wondering how Facebook will cash in on its booming popularity and capitalize on its ad-delivery alliance with US technology giant Microsoft.

Microsoft paid 240 million dollars for a 1.6 percent stake in Facebook in October, giving the young company a theoretical value of 15 billion dollars.