A new study from Satmetrix provides some new numbers on how customer evangelists can help grow your business. And how customer vigilantes can hurt it.

The study examined customers in the computer hardware industry and found, using the Net Promoter
methodology, that "promoters" would spend about $1,818 of their own money and refer an
additional $816 of revenue from friends and associates.

That means a customer evangelist helped inspire sales that was nearly equal to 50% of what she'd purchased.

Some quick math shows the potential of customer evangelists more dramatically: Let's say a business has 1,000 evangelists whose average purchase during a period of time is $1,818. That means the group will have spent $1.8 million of their own money and referred $816,000 of business for a total of $2.6 million. Very quickly the cost of goods sold begins to decline while gross margins go up.

Satmetrix found that detractors (or customer vigilantes, as we like to call 'em here) spend a little less, but their bad buzz caused lost sales amounting to $1352 per detractor, nearly negating any of their actual value as a customer.

The bottom line with evidence like this: diligent listening is key. It's imperative to know whether customers are referring you or bad-mouthing you.

The talkative ones are the leading indicators of your future.