I just read an interesting article in the Washington Post about getting banned from Facebook for using it too much. That's right. If your organization is using Facebook, excess legitimate activities (too many messages, too many pokes, too many wall writings, etc.) could get your account suspended or deleted. If your organization uses Facebook for promotional purposes, read the article and maybe check out the post on Get Satisfaction, 13 Reasons Your Facebook Account Will Be Disabled, that they reference in the Washington Post. Some of these points are outdated (I think you can friend over 5,000 people now), but they are all good guidelines in terms of Facebook usage.
Update/New Topic (9/5/08)
In addition to overusage, it is worth mentioning another thing to consider when using Facebook. There have been a glut of recent worm attacks targeting Facebook users. Details about the initial round of attacks and how they were executed can be found at TechCrunch. Though the attacks started in early August, there are indications that they may still be happening. Thanks to Amanda Beals at MainStreet.Com for the topic suggestion and a link to suggestions on how to protect yourself from the hidden dangers of Facebook.