Vodafone found itself the latest victim of a social media hijack this weekend, as online protestors swamped the group’s Christmas Twitter campaign with criticisms of Vodafone’s tax affairs.

As part of its 12 days of smiles competition, the mobile phone operator had promised to give away free handsets to selected Twitter users who used the tag #mademesmile to tell the company what made them happy. But the tag was soon being used to discuss the company's alleged tax avoidance and had become wildly popular. After a relatively quiet start on Saturday, it became the most discussed Twitter topic in the world.

Orchestrated by anti-cuts group UK Uncut, the hijack saw thousands of comments mocking Vodafone. “You dodging SIX BILLION and now looking stoopid #mademesmile,” said @18thBrumaire while @ABHewitt posted, "Vodafones epic social media fail #mademesmile."

More embarrassing was the fact that every tweet containing the hashtag was also being posted, unmoderated, onto Vodafone's website, exposing the controversy to a whole new audience.

Some social media observers have questioned the wisdom of the #makesmesmile initiative. “Now social media has become a mature discipline with a substantial body of best practice and experience to draw on, it’s surprising that many companies still make basic mistakes,” said Stuart Bruce, CEO of Wolfstar Consultancy, a PR firm and social media consultancy. “I’d question if it was even right for Vodafone to attempt this marketing stunt.

“When your brand is already under substantial public attack, it’s asking for trouble to appear to ignore them and instead launch a lame marketing initiative. There were already substantial online conversations about Vodafone and many of these were angry voices, which Vodafone appeared to be doing little to address.“If it had the resources to pay for a marketing campaign, it should have also had the resources to respond to its critics. Surely this was an opportunity for senior Vodafone executives to engage in genuine online dialogue with its stakeholders and customers? Social media is far too important to a company’s reputation to be left solely to marketing people.”

Aileen Thompson, Vodafone UK’s director of corporate communications, declined to comment on the social media campaign but reiterated a Vodafone statement saying: "It is completely untrue to say that Vodafone has an unpaid £6bn tax bill. HMRC itself has called that figure an 'urban myth'. Vodafone has not been 'let off' any tax.”

Much has been made of the terms and conditions of the 12 days of smiles competition with the wording of clause 11 being particularly ridiculed: "Check to see if there's any tax implication by winning a prize as you'll be responsible for this."