Li Evans breaks down the eight most prevalent types of social media users, and how you should direct your content toward, around or away from them. The most valuable users are the brand evangelists, promoters and co-creators. Each group has varying degrees of affinity for a brand--with the evangelists being die-hards who are likely to tell everyone they know, the promoters being less crazy (and more trusted) by friends and family, and the creators getting the word out through their blogs and profiles.
On the other side, you should try to avoid the trolls and rabid detractors at all costs. As evidenced by their names, these types either are a waste of marketing time and ad spend--because they'll never pass the message on or buy, or are outright brand haters, and will do whatever possible to tarnish your name.