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The privately held company has signed on several dozen partners for the system, including movie rental chain Blockbuster and online auction market Ebay.

"What we're building here is a massive network of real world connections through which people can share information," Facebook Chief Executive Mark Zuckerberg claims.

Facebook's new advertising system will test how willing users are to accept advertising in the most personal of media formats. Partners said users would soon make it clear what kind of advertising they wanted to hear. "It's not about being commercial, it's about being useful," says Quincy Smith, president at CBS Interactive.

Zuckerberg was unapologetic when asked whether the influx of advertising could put off users who flocked to the site as a fun way to interact with friends. "It's a free service and it's ad supported," he says.

The new "Facebook Ads" system will allow users to have their online behaviour, including advertiser sites they visit or the products

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they buy, appear on their news feeds if they choose.

"It's way more organic and natural to have advertising based on what your friends are doing," Zuckerberg claims. "We're going to help your brands become part of the daily conversations that are happening every day."

Advertisers that join the program can also pay for a small ad for their product to appear any time it is mentioned on a news feed. Such ads appear on a personal page that only the user sees, and not on their public profiles.

In addition, Facebook claims it had launched more than 100,000 new pages on its site, representing some of the world's largest brands, local businesses and organisations. Facebook is also launching a system for advertisers to home in on audiences they want to reach through a set of keywords.

The new ad system works separately from its partnership with Microsoft, which serves up banner and other display advertising on Facebook pages. Microsoft took a minority investment in Facebook last month that valued the privately held company at $15 billion (£7.19 billion), largely on the expectation the social network would soon mine user profiles for valuable ad data.

Facebook's ad partnerships could pose a new challenge to News Corp's MySpace, the world's largest online social network, and web search leader Google. Both seek to offer advertisers more direct and efficient ways of reaching audiences on the web and have announced new advertising and social network programs in the past week.