Facebook has said it wants to let users advertise products on behalf of companies, in a move which will significantly increase the number of corporate messages on the site.

Members of the social networking site will be given the opportunity to alert people they know when, for instance, they buy a product from another website, in which case their friends would receive a message with an advertisement attached.

Advertisers will pay for the privilege of having their product referred by one user to another, which will work a little like old-fashioned 'word-of-mouth' marketing.

If, for instance, a Facebook user downloads a film from Sony's website, he or she would be given the option of broadcasting the purchase to friends, who would have no choice but to receive the message, along with an ad from Sony.

Users will not get paid for their role as 'brand ambassadors', but the ads will tie into one of the site's main features - a stream of messages that constantly updates friends about each other's actions known as a 'News Feed'.

More than 60 advertisers, including Coca-Cola, Blockbuster, Microsoft, Sony, Verizon and The New York Times have signed up to take part in the new programme, which is due to start this week.

Announcing details of Facebook's new advertising strategy, Mark Zuckerberg, the company's 23-year-old chief executive, said: “Nothing influences a person more than a recommendation from a trusted friend."

He said that the traditional model of advertising, where companies pushed messages out to customers via large media like newspapers and television was changing, and that marketers would become increasingly able to relate more directly to consumers via social networks.

Companies will, from this week, be able to set up 'profile pages' on Facebook where customers can interact with them, Mr Zuckerberg said. They will also be able to take advantage of the rich trove of personal information Facebook has gathered about its users, which now number more than 50 million, in order to target their ads more effectively.

Like MySpace, which earlier this week announced that companies will be able to serve 'hyper-targeted' ads based on information in a user's profile, Facebook offers great promise for advertisers.

Privacy advocates, however, have expressed concern about the site, saying that advertisers may gain access to too much information about people's online behaviour.

Facebook has said that it will share only information that users choose to make public, and that no details which would personally identify a person to a company would ever be passed on.

Analysts predicted that there would be an increased amount of 'brand activity' on Facebook, as marketers jostled to get users to sign up to discussion boards and other services. In a note, Forrester said: "Expect a lot of noise to be generated. Brands will be working to earn and buy fans to accept them as members."

Chris Winfield, president of 10e20, a social media marketing company, said that advertisers would welcome the new platform, but that Facebook risked alienating users if their profiles became too cluttered with marketing.

"Part of the reason Facebook has been so popular is because it's been anti-advertising, anti-clutter," he said. "This risks friends falling out with each other. If someone is constantly telling me how great Coke is and I'm a Pepsi fan, I'm going to lose that connection."

Users, meanwhile, were divided about the new initiative - some saying it was right for Facebook to help companies serve more relevant ads, others warning that the site's reputation as a commercial-free zone would be jeopardised.

"I'm not a privacy hype, but this just feels like hijacking and commercializing personal conversations," read one comment on the technology news site TechCrunch.

"Is this Facebook or Amway?" another asked.

Mr Zuckerberg said the new platform would make the site "less commercial", because advertising messages from come via friends, not from companies directly.

Asked whether people might find the new ads annoying, he shrugged, and replied: "I mean, it's an ad-supported business."

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