• Sweden's Rebtel follows Skype with low-cost calls

Andreas Bernström, head of Swedish mobile calling firm Rebtel, told Reuters that a simple proposition like this could lift Facebook's annual revenue by $800 million and give the social network a direct-billing relation with millions of users.

Facebook voice service charging for calls going into traditional telephones could in theory create the world's second largest telecoms network by customers, after China Mobile and ahead of Vodafone, CEO Andreas Bernström said.

Europe's traditional phone companies, already under pressure from cheap internet-based services like Skype, are about to face more competition from a group of Nordic start-ups that will offer low-cost mobile calls and other services.