4. Falling for the "I'm an old friend you haven't seen in 20 years and I'm stranded in London could you please wire me money" scam. Amazingly, the London Scam actually works. It happens when the fraudster hijacks your old friend's account and then uses it to send out desperate (and convincing) pleas for help. A friend of mine was targeted by this scam and almost bit; a woman in Missouri did bite and found her wallet $4000 lighter.
5. Slagging on your boss, co-workers, friends, or significant others. Listen, we all got gripes. But in the meatspace they tend to dissipate over time; on Facebook they're forever, or as close to it as the digital world generally gets. They may fall harmlessly into the digisphere or they may come back to cause you a world of pain -- there's just no way to know.
6. Being duped by malware. These are typically spread via outrageously titled wall posts like "Justin Bieber just got erection in public" or "Distracting beach babes." Curiosity gets the better of you and you click on the link. Instead of a video of Justin's engorged manhood or bouncing bikini Betties, you get a bad case of Koobface or some other socially borne malware. Worse, that post now becomes part of your wall, tempting your friends to click. If it's too outrageous or salacious to be believed, assume it's malware and move on.