Not all fans are equal and, indeed, not all brands must turn to Facebook to drive their business. No matter the brand, however, the question of the return on investment and the value of a fan are central to any decision to participate and invest in a social media strategy.
The short story is that there is no way to place a single value on a fan. The answer to the question: "What is the value of Facebook Fan?" is that "it depends." Not only are some fans worth more than others, but the way you build up engagement on your page makes the value of a fan more or less potent. This means identifying, for example, who are the major (offline) purchasers, who are the most influent and who are the most engaged. With each category a different type of treatment should be considered.
The good news is that there are increasingly more sophisticated tools to help understand the fan base and who are the more valuable among the fans. It's highly necessary, as the reality of "big data" becomes more accessible, to have a great analytics team, able to collect, collate and render intelligible and relevant that information.
While this infographic is directed toward Facebook, the concept is equally valid for any social media strategy and other social media platforms (Twitter, Google+...). In any event, ultimately, the value of your fanbase depends on how much you truly care about your customers.

Infographic by Minter Dial (The Myndset), Renaud Ramaud (designer) & YOPPS Agency. First published here on The Myndset. If you care to download the PDF version, it's available over on Slideshare here.