SAN FRANCISCO - Saying it went too far in its pursuit of profit, the popular Internet hangout Facebook Inc. is allowing its 55 million users to permanently turn off a new marketing tool that tracks their activities at other Web sites.

The privacy control, announced yesterday in an apology by Facebook founder Mark Zuckerberg, will likely limit the reach of an application called "Beacon."

The tool is part of a month-old program that the Palo Alto-based startup had hailed as an advertising breakthrough.

Facebook users attacked Beacon as a flagrant violation of privacy. The tool enables Facebook to track its users' purchases and actions at dozens of Web sites and then broadcast the data on the pages of their listed friends within its social network.

"We've made a lot of mistakes building this feature, but we've made even more with how we've handled them," Zuckerberg wrote on Facebook's blog.

"We simply did a bad job with this release, and I apologize for it."

Empowering users to block Beacon entirely "is a big step in the right direction, and we hope it begins an industrywide trend that puts the basic rights of Internet users ahead of the wish lists of corporate advertisers," said Adam Green, a spokesman for the advocacy group MoveOn.org.

More than 65,000 Facebook users signed a petition that MoveOn organized against Beacon. *