Privacy advocates declared victory on Friday after Facebook, the social networking website, moved to placate users concerned about the intrusiveness of its new Beacon advertising system.

Changes to Beacon, announced late on Thursday, will allow users to “opt-in” to sharing information through the service, which broadcasts purchases made on outside websites to Facebook users’ friends.

This is the second time in a little more than a year that Facebook has been forced to change a new technology after a backlash over privacy. MoveOn.org, the online advocacy group that last week joined critics of the Beacon service, called Facebook’s move a “big step in the right direction”.

Beacon has proved the most controversial of several new money-making technologies unveiled last month by Mark Zuckerberg, Facebook’s 23-year-old founder.

The technologies represented its most ambitious attempt yet to cash in on the social connections between the site’s 50m users.

Under the service, friends of a Facebook user who buys a book on Amazon.com, for example, may see the Amazon logo next to a message about the purchase when they log into the Facebook site. The changes mean that users will now be asked to explicitly authorise the publication of each Beacon message, according to Facebook.

Previously, messages would publish automatically unless a user said “no” to a publication request within a certain amount of time.

Facebook also introduced the ability to opt out of the Beacon service on a website-by-website basis.

Charlene Li, an analysts at Forrester Research who had complained about the service, applauded the changes. But she said the site had to “do a lot more lot more to regain the trust of not only its members but also its partners,” some of whom are nervous about a further privacy backlash.

“I believe some privacy advocates will continue to agitate for the full dismantling of the Beacon program,” Ms Li said.

Facebook faced a similar backlash last year when it launched Newsfeed, which alerts friends to everything a user does on the site. Newsfeed went on to become one of the site’s most popular features after the company tightened privacy controls.