Faced with its second mass protest by members in its short life span, Facebook, the enormously popular social networking Web site, is reining in some aspects of a controversial new advertising program.

Within the last 10 days, more than 50,000 Facebook members have signed a petition objecting to the new program, which sends messages to users’ friends about what they are buying on Web sites like Travelocity.com, TheKnot.com and Fandango. The members want to be able to opt out of the program completely with one click, but Facebook won’t let them.

Late yesterday the company made an important change, saying that it would not send messages about users’ Internet activities without getting explicit approval each time.

MoveOn.org Civic Action, the political group that set up the online petition, said the move was a positive one.

“Before, if you ignored their warning, they assumed they had your permission” to share information, said Adam Green, a spokesman for the group. “If Facebook were to implement a policy whereby no private purchases on other Web sites were displayed publicly on Facebook without a user’s explicit permission, that would be a step in the right direction.”

Facebook, which is run by Mark Zuckerberg, 23, who created it while an undergraduate at Harvard, has built a highly successful service that is free to its more than 50 million active members. But now the company is trying to figure out how to translate this popularity into profit. Like so many Internet ventures, it is counting heavily on advertising revenue.

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