The trade-off between functionality and privacy. Lifehacker's post is a good summation, and includes steps to protect your Facebook privacy.
My feeling is that anything sent to the internet risks becoming available to people who you never want to see it. I cross my fingers and trust in encryption when I buy things online. For the rest, what I share I try to run past a mental filter--in 10 years time will I cringe if this reappears? (Not that anyone's going to bother holding anything against me, I'm an ant in a 6 billion world)