The service, called Beacon, feeds information about users' online activities at partner sites into a stream of "stories" made visible to other members of a user's social network. It also allows the partner sites to tack a "social ad" containing the user's photo onto the story, suggesting that the user endorses the partner's services.

Actions that can be broadcast in this way include purchasing a product, signing up for a service, or adding an item to a wish list. One of the first partners to use the service was Blockbuster: when a Facebook user added a movie to their queue on the blockbuster.com website, that information was transmitted to their Facebook contacts.

Last night, Facebook said it would ask users to opt in to the service before broadcasting details of their online activities to others. A separate opt-in question will be displayed on their Facebook profile page for each of the partner sites they use, it said. If they do not reply then the question will disappear after a time, only to reappear each time they use the partner site until they either allow or refuse the broadcasting of messages about their activity there.

When Beacon launched earlier this month, Facebook displayed an opt-out message at the end of each transaction on a partner site, inviting users to click if they didn't want details of their purchase or activity broadcast to their friends.

Facebook acknowledged that the initial opt-out messages were not displayed clearly enough or soon enough after the transaction. Users were sometimes moving away from a partner page before a notification could be fully displayed, Facebook said.

The company's assumption that users had seen and agreed to the notification by default angered online advocacy group MoveOn.org Civic Action, prompting it to set up an online petition against Beacon.

Although Facebook circulated information about its U-turn to news outlets, it has not yet informed its users directly of the change through either its corporate blog or the section of its Web site devoted to news releases.