If you are an unhappy customer: your friends.
A new study reports that irritated customers are five times more likely to vent to a friend than a store rep and on average they will tell four friends. Why should you care? The same study reports that those told about a friend's bad shopping experience are up to five times as likely to avoid the store in question as the original unhappy customer!
Did you get that? For every disgruntled customer that walks out your door there will be four potential customers that are five times less likely to give you a try.
The solution: obviously make sure each customer experience is great and then proactively ask your customers about their experience. If they had a bad one, turn it around and do it quickly.