The first thing you need to do in order to be a success with any social media outlet is to define exactly what it is you are trying to do (gain sales, members, fans, whatever). Then design an approach to "solve" your issue. In doing this, you must decide on an approach that gives your target information that they are truly interested in. You must ENGAGE them in conversation. The very worst thing you can do is talk about YOURSELF, or your product, or push a sale. Engage and establish yourself as expert!
I can give you an example of what I have seen several companies do. They will hold contests on FaceBook for their current fan base, utilizing them to build the fan base..."get three of your friends to like us on FaceBook, and we will send you a gift certificate"..or something to that affect. Certainly you can see the power in this "snowball"effect of gaining friends/fans, and also the potential for additional via not only the winner of the contest, but others that have now fanned you.
Fridays Restaurant chain did a promo recently where they sent every person who fanned their site a free burger. While this probably cost the company some cash, how many people go to Fridays and get just that hamburger? I would bet a large percentage also buy a cocktail...or fries..or whatever. Clearly this is a measurable promo,also!
Too many companies take a "set it and forget it" approach which does not work wtih Social Media. My Mother always taught me that there is no such thing as free in this world. Social media appears to be free, but it is not. It takes time and effort to be successful with Social Media marketing, just like it does with any marketing endeavor.