When Mark Zuckerberg walked away last year from a reputedly $1bn (£493m, €725m) offer for Facebook, the social networking site he had launched from his Harvard dormitory two years before, some predicted it was a decision that the 23-year-old internet prodigy would come to regret.

It was less than a year since RupertMurdoch's News Corp had bought MySpace, the world's biggest social network, for a then eye-popping $580m. That deal sparked a frenzy of interest in websites that allow people to find each other and share messages, photographs and other personal information online.