The system Facebook introduced this month, called Beacon, is viewed as an important test of online tracking, a popular advertising tactic that usually takes place behind the scenes, where consumers do not notice it. Companies like Google, AOL and Microsoft routinely track where people are going online and send them ads based on the sites they have visited and the searches they have conducted.

But Facebook is taking a far more transparent and personal approach, sending news alerts to users’ friends about the goods and services they buy and view online.

Charlene Li, an analyst at Forrester Research, said she was surprised to find that her purchase of a table on Overstock.com was added to her News Feed, a Facebook feature that broadcasts users’ activities to their friends on the site. She says she did not see an opt-out box.

“Beacon crosses the line to being Big Brother,” she said, “It’s a very, very thin line.”