I've been on Facebook for a while now, and while I have broken some of the following rules, I've learned from my mistakes. Take it with a grain of salt, but following is my take:
1. Do not post anything that you don't care to have repeated, even if you think your status is secure amongst your closest friends.
2. Speaking of close friends: if you plan to socialize with a few of them, but not all of them, please do not post; it will hurt people's feelings. Besides, life isn't a competition.
3. Whatever you do, do NOT post if you are home alone at night, where you are going specifically, or who is babysitting your child(ren) is. Common sense = safety. Also, don't post your address (which I just did by accident).
4. Please don't post something random, i.e., "I'll never get over this..." or "I have a secret but I can't tell yet..." without an explanation. This drives your friends (and those that aren't really friends) bananas.
5. Always pretend that your boss and most respected family and friends can see what you post, even if you are sure that they can't. This way, you'll never embarrass yourself (or get in trouble).
Let's keep in mind that everything I've listed has either a) happened to me, or b) something I'm guilty of doing to someone else. I certainly don't think I'm perfect. I'm learning. And growing up (albeit slowly), and while I'll NEVER be perfect, at least I can learn as I go. And hopefully, someday, I won't hurt anyone, ever. At least intentionally.
This post ended up being a bit deeper than Facebook etiquette, no?