Some users of the online hangout Facebook are complaining that its two-week-old marketing program is publicizing their purchases for friends to see. Those users say they never noticed a small box that appears on a corner of their Web browsers following transactions at Fandango, Overstock and other online retailers. The box alerts users that information is about to be shared with Facebook unless they click on "No Thanks." It disappears after about 20 seconds, after which consent is assumed. Users are given a second notice the next time they log on to Facebook, but they can easily miss it if they quickly click away to visit a friend's page or check e-mail. "People should be given much more of a notice, much more of an alert," said Matthew Helfgott, 20, a college student who discovered his girlfriend just bought him black leather gloves from Overstock for Hanukkah. "She said she had no idea (information would be shared). She said it invaded her privacy."