Facebook will change a recently introduced advertising tracking program after 50,000 users signed an online petition and some complained that their holiday shopping purchases were made public without their permission.

In an announcement made last week, the popular social networking site said it will now ask for explicit consent before publicizing recent online purchases through the “News Feed,” a function that allows Facebook friends to follow each other’s activities.

Before Facebook amended the tracking function, users found their gift ideas spoiled because they had not known to disable the feature each time they bought something.

Facebook’s social advertising feature uses Beacon, an online tracking system which displays ads of participating companies from which users have recently made purchases. The system also alerts users’ friends to new purchases.

Though the tweaks to the feature may have appeased most users, some users said they are still worried about their privacy.

“With the nature of Facebook, privacy is a fine line,” said Jeffrey J. Lee ’11. “But some things need to be limited, like the shopping thing,”

Adam Green, a spokesman for MoveOn.org Civic Action, the political group that created the petition, said in an e-mailed statement that “Facebook’s policy change was a huge step in the right direction, and we hope it sets an important precedent in favor of Internet users’ rights that the entire industry follows.”

Green said that MoveOn.org has not decided whether it will demand a complete opt-out, whereby Facebook users will be able to disable the ad tracking feature once and for all.

Facebook faced a similar large-scale outcry from its users when it introduced the “News Feed” last year, but it has since received positive feedback about this feature.

The information shared is based on “conversations that are already happening between people,” Facebook founder and CEO Mark E. Zuckerberg, formerly of the Class of 2006, said when introducing the program in New York last month.

But some users might need more time.

“Why would my Facebook friends want to know what I buy? ‘Hey pals, look what I brought on Amazon!’” Daniel J. Thorn ’11 said.

Entrepreneurial legal studies professor Yochai Benkler wrote in an e-mail that he sees Beacon as “Facebook’s effort to take that most powerful form of recommendation—‘hey, that’s cool, where did you get that?’—and instantiate it in the technical platform.”

“But what we saw was that the same force—the social network—that made the recommendations so valuable also provided the basic political architecture to force the company to back down,” Benkler said.

A Facebook representative did not return an e-mailed request for comment.