In a word… Privacy.
As of December much of your profile is now public. As much as it’s great to connect with friends online, how about all of their friends as well just of a degree of separation. How about your name being searched in Google and your name, city, and your other profile information being shared by default because Facebook changed their privacy rules despite 14 different privacy advocate groups filing complaints.
Know what Facebook is doing and just who owns the information you post online.