Ask this question before thinking why you hit southwards in your balance sheet. It is not that your product is worth nothing or your market strategy is bullshit. How about this proposition: Are you targetting the right customers? In this media-frenzy society where everyone is connected somehow, either via SNS like Facebooks, MySpaces etc or through Twitter, Friendfeed blah blah, so how can you think that your customers are not influencing each other. They are!
Look at the proposition above. When smart & interesting people talk about you or your product/brand par se, it generates something invaluable which your stupid ads doesn’t since they are more often than not slapped on the customers face when they never wanted it in the first place. But targetting those invaluable customers who wants to listen about you makes sense since he will do the last bit of word of mouth for you.