Nothing's more powerful for marketing than the Word of Mouth. When you reach a point when a single user recommends your product to more than one of his friends, your business tips. But not all words of mouth (or is it word of mouths?) are created equal.
Some are pushy ones - like a person using a "Tell a friend" feature. Its like selling Amway products, more like "Spam an Enemy". All social networks are guilty of these.
Next up are sneaky ones - seemingly innocent but highly disgusting. Most photo sites are guilty of these - "Sign up to see your friend's pictures". The user invites a friend to see photos and you gotta sign up to just see them (and in turn give them permission to spam you for the rest of your email life). Facebook apps, Plaxo's spam - are all great examples. Its like inviting a friend to your wedding and charging a cover fee at the door. No shit! The sad thing is, they work... but are laden with risks.
Next up are actual word of mouths. Its the "real" deal. A user loves your product, talks about it, endorses it and almost sells it for you. Everyone wants a word like this, but they are inherently risky. Users hype up the expectations and the friend can only be disappointed. Ever got a movie recommendation that was just not as good as it was hyped up to be?
And the last ones are "whispers" - these are not active referrals. In fact they aren't referrals at all. You just see your friend using it and love it. The iPod is a classic example. Blogs are another. Evite, FirstPhera, your new shoes, your shirt, your car - are all great examples.
While building a product, don't go for the bang. Try get the whispers!