The world's largest social networking sites have gone head to head in a battle to win over advertisers by offering them access to narrowly defined target audiences.

MySpace today announced details of a new platform which will let companies deliver adverts to users based on interests they express on their public profile - movies or sport, for instance.

Facebook, meanwhile, is preparing to announce details of its new platform tomorrow. Reports suggest that it will give users the option of letting the site track their movements on the web at large.

Social networking sites offer enormous promise to advertisers because of the vast amount of information entered by users on their profile pages - including geographical information, hobbies, employment and other details.

There have also been fears that personal information intended to be kept private could be used by the networks for advertising purposes, which has led MySpace to assure users that only the details they choose to display publicly will be used.

As part of its new platform, MySpace, which is owned by News Corporation, the parent company of Times Online, will divide its users into more than 100 categories with names like 'movies', 'travel' and 'auto', based on information in their profiles. Advertisers will then be able to target campaigns at people who are more likely to be interested in seeing them.

For example, if users said they supported the Boston Red Sox and listed sports movies among their favourites, they would be entered in the 'sports' category, making them an ideal candidate for sports-related advertising.

As well as these broad groups, MySpace is also compiling lists of people with more specific interests - for instance, within movies, those who like horror films - to enable an even greater degree of relevance for a particular advert, a service the company calls 'hyper-targeting'.

The more specific the information that an advertiser elects to make use of, the more it will pay.

Chris de Wolfe, the chief executive of MySpace, said that he hopes to expand the number of categories - or what the company calls 'buckets' - to more than 1,000 in the coming months.

“The thing we heard from (users) is they wanted advertisements to be more contextually relevant to them,” Adam Bain, executive vice president for products and technology at Fox Interactive Media, said. “If they have to live with ads, they want an ad to be engaging.”

He added that MySpace, which has more than 110 million users, had spent a year developing the program and had used focus groups to study issues such as privacy.

Tomorrow's announcement from Facebook, is also expected to concern the targeting of advertising based on information within a user's profile.

Reports suggest that the site is considering giving its users the option to let their activity on the web at large by a tracked. The idea is that information gathered about their movements - purchases from other sites, for example - could be shared with friends.

In a separate development, MySpace said it was releasing a set of 'self-service' tools that will enable smaller advertisers not targeted by the company's sales teams to create and place their own ads on the site.

* Yahoo! today announced details of a new social networking service that will enable university students to tap into a network of professionals and get advice and internships and job offers.

Kickstart aims to benefit young people who don't yet have extensive professional contacts but who use other networks - such as Facebook - primarily to communicate with friends, rather than further their careers, Yahoo! said.

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