Two weeks ago, an e-mail landed in the inbox of Ben Stiller, the Hollywood actor. With the release of Tropic Thunder, an upcoming action comedy, would he allow his image to be used in a computer game to be distributed on Facebook?

The social networking site is well-known as a way for its 80 million members to keep in touch, but it is only beginning to assume a new guise as a gigantic viral marketing platform. And companies are lining up.

The Tropic Thunder game, designed by Techlightenment, a London-based digital agency, is a “shoot-em up” that taps into the Paramount film's mock-heroic strain of humour - players are rewarded for overly dramatic acting.

If this sounds frivolous, consider the companies that are spending a part of their marketing budget on applications, or “apps” - small computer programs that can be downloaded and shared between friends.

Disney has commissioned a Facebook app for Wall-E, the Pixar film about a rubbish-collecting robot (there are ten levels; players collect trash). For the release of The Dark Knight, Warner Bros commissioned an application that lets users give their photos a Batman-esque flavour.

For Juno - whose plot centres on an unplanned pregnancy - Fox Entertainment created an app that allowed users to send virtual pregnancy tests. In the three weeks of the campaign, Facebook users sent more than three million virtual tests, according to Slide, the app's creator.

According to a report by Forrester, the total spend on “interactive marketing” is predicted to more than triple from $18 billion (£9.1 billion) to $61 billion by 2012.

Of the $61 billion, by far the largest chunk will be spent on search and online display advertisements, which between them will account for $40 billion, yet social media apps are expected to grow from $1 billion to $11 billion in that period.

Many remain sceptical about the earnings potential of sites such as Facebook. When Microsoft bought a $240 million stake that led to the site being valued at $15 billion, Kara Swisher, the influential Wall Street Journal columnist, wrote that Facebook was like a “lemonade stand” in comparison with Google, to which it is sometimes compared.

The move to new approaches is driven in part by a realisation that banner adverts across the tops of web pages have fared poorly on social networking sites.

Sonya Chawla, managing director of advertising at Slide, said: “If you look at click-through rates on traditional ads in social media environments, they're extremely low. Users just don't respond well to ads pushed into their [personal] environments.”

Simon Mansell, managing director of TBG, a London-based digital agency, said: “It's the equivalent of having an nPower ad thrust at you while you're in the pub.”

By comparison, click-through rates on ads that accompany, or are part of, a game are, by one estimate, 20 times higher at 1 per cent, rather than 0.05 per cent. Advertisers are also enticed by the prospect that campaigns on social networking sites can be tracked more easily.

It is not just social networks that have attracted marketers. Last month Apple unveiled its “App Store” - an online catalogue of apps for its portable devices. One early success has been the iPint, a Carling-themed game developed by Coors, the brewer.

Not to be outdone, Google will reveal its Android mobile phone operating system this summer. It will mean that millions more mobile customers, who buy handsets made by the likes of HTC, Motorola, and Samsung, will be able to download apps to their phone, and make themselves available to marketers.

Stiller, by the way, writer, director and star of Tropic Thunder, a war movie spoof, answered the Facebook question with a “yes”.

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