Facebook plans to allow users to advertise products on behalf of companies in a move that will increase significantly the number of corporate messages on the site.

Users 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 will receive a message with an advertisement attached.

Facebook users will not be paid for their role as “brand ambassadors” but the adverts will tie into one of the site’s main features – a stream of messages known as a “newsfeed” that constantly updates friends about one another’s activities.

Advertisers will pay for the privilege of having their product referred by one user to another, which will be akin to word-of-mouth marketing. If Facebook users download a film from Sony’s website, they will be given the option of letting their friends know in their messages, which will include a Sony advert.

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

Mark Zuckerberg, 23, the chief executive of Facebook, said: “Nothing influences a person more than a recommendation from a trusted friend.” He noted that the traditional model of advertising, using media such as newspapers and television, was changing. Increasingly, he suggested, marketers would be able to relate more directly to consumers via social networks.

Advertisers will be able to set up profiles on Facebook that will enable customers to interact with them, Mr Zuckerberg said. They will also be able to take advantage of the rich trove of personal information that Facebook has gathered about its users, who number more than 50 million, to pinpoint their commercial messages.

Like MySpace, which announced this week that companies would be able to serve “hypertargeted” adverts based on information in a user’s profile, Facebook offers great promise for advertisers. However, privacy advocates have expressed concern, saying that advertisers may gain access to too much information about users’ online behaviour. Facebook has said it will share only information that users choose to make public, and that it will not pass on details that identify individuals.

Analysts predicted that brand activity would increase on Facebook as marketers jostled to get users to sign up to discussion boards and other services. Forrester said in a note: “Expect a lot of noise to be generated. Brands will be working to earn and buy fans to accept them as members.”

Chris Winfield, the president of 10e20, a social media marketing company, said that advertisers would welcome the 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 antiadvertising, anticlutter,” he said. “This risks friends falling out. If someone is constantly telling me how great Coke is and I’m a Pepsi fan, I’m going to lose that connection.”

Some users say that Facebook is right to help companies to serve more relevant adverts but others believe that the site’s reputation as a commercial-free zone is under threat.

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