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.