The New York Times reports on an internship candidate that lost his opportunity through a (probably completely fictional) Facebook profile that featured his love of marijuana, obsession with sex, and affection for shooting people. But the lesson here is probably not "don't make a Facebook profile" (or MySpace, or etc.), but rather "your online persona is the only integrated persona you have, so make it real."
Which is really to say, the only media that is safe to have in a networked application, where information is effortlessly accessible and portable, is authentic media. The very nature of the network makes it certain that every past mistake, embarrassing problem, or secret wish will come out. You can't have different personae for work, home, friends: you must be humble and honest and create something as close as possible to yourself.
Update: Oops, Mom Googled Me