Before jumping into social networks, it's important to take a look at the numbers and realize that there are a lot more failures than successes. The following chart makes it pretty clear -- there are a few highly successful group pages, but even within the 10 largest, the drop in membership is exponential. After that, the numbers become much smaller and stretch out into a long tail of obscurity. 

It is essential that brands attempting to foray into the social networking space understand that the key demographic is fickle, there is no guarantee of success, and best practices are at times nebulous and highly dependent on the individual brand and project goals. A good first step is to search for brands with similar demographics, or goals, and see what they are doing on social networks and how they are faring with their attempts. Also, do some serious soul searching.
Are you embarking on a social networking campaign because someone thinks it's cool, or because it fits in with a larger approach? Think of it like any other marketing channel -- if it's not part of a larger holistic strategy, it's likely to fail.