Can some explain the etiquette of social websites

I dont seem to understand the etiquette of social websites; such as facebook and etc. I have friends from high school, friends I met along the way in life, some marketing networking and such on my friends list. I am a little conscience on who I add to my page. If you request my friendship and I dont know you, I will be a little kick-picky on whether I will accept that request. I will not just add if you want to JUST increase your friend's list. I mean marketing certain services are different. If you have a business and you are looking to expand your marketing through facebook and its not against who I am, sure why not. Does that make sense? That is not the point of this blog so I will get to the point. 

What do you expose on your social website? I am amazed on how many people will talk about their issues in marriage, the hate of family members, and just stuff that a stranger shouldnt know. Am I overlooking the point of this. I believe that the world should not know that you husband is a dick and you are thinking of leaving him. We are friends because you have an awesome home-based business not on a personal level. Did you forget that you add friends that do not have an history with you.

 I met someone through my pregnancy and she found me on facebook. It was nice to talk to someone that I knew when I was pregnant with my girls. One day she is going off on people. How they are taking what she said on facebook and using it against her. She stated that they misused her words and using it against her. I personally think that she gave the ammo for others to use. Then I got a new friend request with a note attached that she is creating a new page for people that she trust. Now, I accepted the request because I dont have anything against her BUT we are not close friends. It was just an association that we had when we were pregnant. I only know the basics about her and that's about it. How can I be someone that she trust when she doesnt "really" know me. 

Now this is my question. When is too much too much for all to read. I know some people need the outlet and use these sites to do so. I even had to email reminders about OPSEC. Some things shouldnt be talked about on these sites especially when you have 500-1000 friends on your page. Most people are close with maybe up to 5-10 people. Someone help me understand, please.