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.

• Click here for FOXNews.com's Personal Technology Center.

"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."

The girlfriend was declining interviews, Helfgott said.

An Overstock.com Inc. spokesman said no one was immediately available for comment Wednesday.

Facebook, based in Palo Alto, Calif, has long prided itself on guarding its users' privacy, but the walls have gradually lowered.

In 2006, a "news feeds" feature allowing users to track changes friends make to profiles backfired when many users denounced it as stalking and threatened protests.

Facebook quickly apologized and agreed to let users turn off the feature.

The new program lets companies tap ongoing conversations by alerting users about friends' activities through the feeds.

About 40 Web sites have decided to embed a free tool from Facebook, known as a Beacon, to enable the marketing feeds.

The idea is that if users see a friend buy or do something, they'd take that action as an endorsement for a movie, a band or a soft drink.

But it also raises privacy concerns.

Mike Mayer, for instance, saw a feed item saying his boyfriend, Adam Sofen, just bought tickets to "No Country For Old Men" from movie-ticket vendor Fandango.

"What if I was seeing 'Fred Claus'?" said Sofen, 28. "That would have been much more embarrassing. At least this was a prestigious movie."

In some cases, companies can buy an ad next to the feed item with the friend's photo. Although Fandango didn't do that, Mayer, 28, still found Beacon unsettling.

"If my identity is going to be used to promote something for someone else, that seems problematic," said Mayer, who was previously employed in online advertising. "It could be a misrepresentation of my purchases."

Fandango officials referred inquiries to Facebook, which issued a statement defending its practices. Facebook officials have also said advertising supports the free service.

"Beacon gives users an easy way to share relevant information from other sites with their friends on Facebook," the statement said. "Information is shared with a small selection of a user's trusted network of friends, not publicly on the Web or with all Facebook users. Users also are given multiple ways to choose not to share information from a participating site, both on that site and on Facebook."

Users are able to decline sharing on a site-by-site basis, but can't withdraw from the program entirely.

On Wednesday, Facebook launched a mechanism for users to indicate what types of news feeds they like and dislike. Individuals could possibly use that to lower the frequency of marketing items, though the company has said they won't be able to reject them completely.

Liberal advocacy group MoveOn.org formed a protest group Tuesday and had more than 6,000 members by Wednesday.

The group is calling on Facebook to stop revealing online purchases and letting companies use names for endorsements without "explicit permission."

"We want Facebook to realize that their users are rightly concerned that private information is being made public," MoveOn spokesman Adam Green said, adding that Facebook could quell concerns by seeking "opt in" consent rather than leaving it to users to "opt out" by taking steps to decline sharing.

Facebook user Nate Weiner, 23, said he uses a tool for the Firefox Web browser called BlockSite, which he says prevents sites from sending data to Facebook.

"What if you bought a book on Amazon called 'Coping with AIDS' and that got published to every single one of your friends?" he said.