A simmering unease about threats to privacy from a new feature on Facebook is threatening to come to the boil, presenting the fast-growing social network with the first test of its unusual plans for making money from its site.
By automatically alerting a user's online network of friends to things bought on other websites, the feature can reveal highly personal information, critics say.
Facebook, however, says its users can choose to keep their purchases secret, or to limit the number of online friends to whom their purchases are disclosed.
Known as Beacon, the feature was one of several money-making ideas Facebook launched this month to try to turn its users' actions - such as their online purchases and their stated preferences for certain brands - into recommendations that might influence the buying habits of their friends.
Though it caused unease around the internet when first announced, the Beacon system has attracted a renewed burst of unwelcome attention in recent days thanks largely to the efforts of MoveOn.org, the online political action group.
The attention has been fuelled further by this week's Thanksgiving holiday, which marks the high point of the seasonal boom in online shopping.
MoveOn launched an online petition calling on Facebook to apply Beacon only to users who have specifically opted in to the system. At present, Facebook says its users are given two chances to opt out of sending a Beacon alert to their friends - when making a purchase on a website that uses the system, and again on Facebook itself.
However, some online shoppers have complained that these warnings have not always been given, or that they are easy to miss.
One analyst, Charlene Li at Forrester, reported that her husband's purchase of a coffee table on Overstock.com had been reported to her own network of Facebook friends, since the computer they shared could not tell which of them had made the purchase. Also, no warning had been given by Overstock, she said.
"The biggest problem is the lack of transparency," Ms Li wrote on her blog. "There's a fine line that gets crossed when behaviour data slips from being a convenience to being Big Brother. This is one of those times."
Facebook faced similar unease more than a year ago when it launched a feature, known as Newsfeed, that alerts friends to everything a user does on the site.
The social networking site responded by giving users more power to limit the items that appear on the Newsfeed, and the ability to restrict who can see it.
Since then, the feature has become one of the site's most popular.