Facebook, the social network under pressure to convert clicks into cash, has angered its users by publicising details of their shopping habits without their permission.

The most serious damage done so far by Beacon, a new advertising system designed to tap into “the recommendation generation", has been to alert people to Christmas gifts that were meant to be surprises. But Facebook members are angry that the tool has notified their online “friends” of purchases made on retail sites outside of the social network.

"Matt in New York already knows what his girlfriend got him for Christmas," said one post on Facebook.com.

"Why? Because a new Facebook feature automatically shares books, movies, or gifts you buy online with everyone you know on Facebook. Without your consent, it pops up in your News Feed - a huge invasion of privacy."

Facebook members using services such as Fandango.com claimed to have missed a box on the tickets web site that gave them the chance to opt out of the Beacon system.

The row threatens to distract Facebook as it strives to reap profits from its vast user base. The three-year-old site claims more than 55 million members and recently achieved an implied valuation of $15 billion after Microsoft took a minority stake. But it is expected to post a profit of only $30 million (£15 million) this year.

EBay, a Beacon member, has argued that communicating to consumers through “the people they already know ... has the potential to be a powerful tool."

Analysts agree: The Future Foundation, a think tank, this week published a report that said social networks will be a force in online retail. It found that the "ultimate endorsement for a product now comes from the 'lips and clicks' of friends and contacts on sites such as Facebook and MySpace, as purchase is more likely to come from recommendation than any other form of marketing."

The report estimated that with £13.8 billion already expected to be spent online this Christmas, retailers who embrace the "recommendation generation" could make an additional £750 million. However, just how social networks can turn a profit and spread commercial messages among groups of acquaintances without upsetting people remains unclear.

Meanwhile, MoveOn.org, the heavyweight US leftwing campaign group, has suggested that Beacon threatens privacy across the web. "Facebook and similar sites have the potential to really revolutionize how we speak to each other in our society," a spokesman for MoveOn.org said. "When people see their privacy violated, it sullies the entire thing."

A Facebook spokesperson said that MoveOn.org was "misrepresenting how Facebook Beacon works".

He said: "Information is shared with a small selection of a user's trusted network of friends, not publicly on the web or with all Facebook users. Users also are given multiple ways to choose not to share information from a participating site, both on that site and on Facebook."

Earlier this year, Facebook shrugged off privacy fears when Chris Kelly, the group's chief privacy officer, told The Times: "We have always said that information [submitted by users] may be used to target adverts."

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