The contest participant
Online contests and sweepstakes are a common way to quickly gain "likes" on your Facebook page and most likely a large portion of your fan base. Many companies have very successfully built large lists of followers this way, but the downside is that many sweepstakes offers are inadvertently designed to attract large numbers of non-qualified prospects. After all, who wouldn't want to win a trip to Hawaii?
Many people will gladly "like" you for great offers whether they care about your products or not. They'll also tell their friends about it as well. So if you've run a lot of sweepstakes, you might have a large list of acquaintances -- but not many close friends. If you're thinking of running a sweepstakes to gain fans, focus on offers that will only appeal to the fans you'd like to have. For example, if you're targeting IT professionals, don't give away an iPad; instead, provide an offer for a free online training course with a relevant certification.
The one-time prospect
One-time prospects are the people who've visited your page for a specific reason and "liked" you once, but they really aren't that interested in you on an ongoing basis. A good example of this might be people who like a resort page for an upcoming vacation, but they aren't likely to come back.
Although not an active audience, this is an ideal set of people to have on your fan list. They might not interact very much, but they might be inspired to revisit your resort when they see posts two or three years later. Of course, if your service starts to slip and you start getting negative comments, they might be more inspired not to return as well.