Tesco and Asda have been hit by a Facebook scam, with customers sent hoax promises of free shopping via a fake promotion that swept the internet.

Both supermarket chains warned shoppers this week after many received messages telling them they had won up to £500 of vouchers. Facebook said it was investigating what could have been an attempt to con users into providing their details to an unscrupulous marketer.

Meanwhile, Tesco has launched a survey of its Facebook and Twitter users as it considers potential changes to its social network strategy.