A comment on a social-networking site or blog made by a disgruntled customer can cost a company as many as 30 customers, according to a survey from Convergys.

Calling it a form of “silent attrition”, Frank Sherlock, senior vice-president for the customer care service provider, said customers who post negative feedback often don’t bother to complain in person to the company.

Although someone will stop doing business with a firm, their comments will often deter other potential clients.

The survey found that a review on a site such as Twitter or Facebook can reach an average audience of 45 people. And Convergys said companies should be aware that these comments have a “definitive measurable impact”, which can hugely affect their reputation; with two-thirds of those reading a negative comment saying they would avoid or completely stop doing business with that company.

A famous example is that of Canadian musician Dave Carroll and his spat with United Airlines over a damaged guitar. He spent nine months haggling with the airline trying to get compensation to no avail.

So he turned his experience into a song and video which he posted on Youtube. The catchy ditty became global success, United Airlines paid $3000 to a music charity in Mr Carroll's name.

Christopher Rawlins, head of strategy for Consumer Focus, the statutory organisation campaigning for a fair deal for consumers in England, Wales, Scotland, and, for postal services, Northern Ireland, said he wanted to see more consumers involved this way.

Mr Rawlins said the organisation’s labs, which build online tools to make people’s dealings with companies better, was already considering a project on how social-networking sites and blogs can help consumers who have complaints.