The popular social networking websiteFacebook just backed down from a controversial new advertisingprogram after a revolt by thousands of members.

Facebook had launchedBeacon, which was using “social advertising” technology to broadcastinformation about online purchases without many users’ consent. The ideawas to convert private commerce into public endorsements: “Ben Bloom ateat the restaurant Junnoon,” read one ad, with a prominent head shot of Bendisplayed next to the company logo. But what if Ben didn’t want hislunch date to be an ad? Beacon enrolled people automatically, offeringusers a choice to “opt out” of each ad on an individual basis.

Many people didn’t like that, so they protested, naturally, onFacebook. MoveOn started a group demanding that Beacon switch to”opt in”–a default to protect uninformed users–and allow people toreject the program completely in one click.

A new group, Facebook: StopInvading My Privacy!, quickly swelled to more than 50,000 members. Itwas a hub for activism, news and stories about Beacon snafus, includingChristmas surprises spoiled by posted ads.

Students from across the country signed up to lead the group as self-declared “privacy avengers,”and its message board drew more than 1,000 posts in less than two weeks.Then Facebook conceded to the first demand, scaling back Beacon so usersmust choose to participate.

MoveOn declared victory, crediting “everydayInternet users.” The partial retreat was especially striking becauselast year, a much larger protest group of 700,000 users did not compelFacebook to abandon the “feed,” a new feature that blasts updates aboutpeople to their personal networks.

This time, however, the activism wasnot limited to decentralized complaints. MoveOn added criticalleadership and a practical reform agenda, while users spread the wordabout Facebook on Facebook.