Online campaign group MoveOn.org has started a protest against a new advertising model being used by Facebook.

The group has set up an online petition and founded a Facebook group called 'Facebook, stop invading my privacy'. To date the group has over 7,000 members.

"When you buy a book or movie online, or make a political contribution, do you want that information automatically shared with the world on Facebook?" the group asked.

"Most people would call that a huge invasion of privacy. But recently, Facebook began doing just that. People across the country saw private purchases they made on other sites displayed on their Facebook News Feeds."

The furore kicked off when Facebook introduced its Beacon advertising service. Beacon is used in conjunction with around 40 e-commerce sites and posts details of users' purchases on their Facebook news board.

Users can click on a Beacon box on the site if they want to keep their purchases secret, but in many cases the box disappears after 20 seconds and consent is assumed to have been given.

Fandango, eBay and Sony are among the sites using the system.

"We encourage feedback from our users on new products. But MoveOn.org misrepresents how Facebook Beacon works," said Facebook in a statement.

"Beacon gives users an easy way to share relevant information from other sites with their friends on Facebook."

But this has cut little ice with MoveOn.org.

"If Facebook argues that sharing private information with hundreds or thousands of someone's closest 'friends' is not the same as making that information 'public' that shows how weak its argument is," said MoveOn.org spokesman Adam Green.

"Facebook users across the nation are outraged that the books, movies and gifts they buy privately on other sites are being displayed publicly without permission. It is time for Facebook to reverse this massive privacy breach."