I do not participate in any of those social sites nor do I ever plan to. I do have access to my teen DD's FB so I can make sure nothing inappropriate is happening.
Here is what I find from watching her FB account:
The teens post one of three things:
1. I'm SOOOOOOOOOOOOOOOO bored. I have the most boring life. Someone friend me because I am sooooooo bored.
2. My life sucks. School is so hard, boring. long, etc. My job is the worst. My parents suck.
3. I am with my BFF and we are having the time of our life watching a movie. BFF and I are going for ice cream now. We are having such a great time. BFF and I are heading to the mall now. Can't wait. It will be so much fun.
#1 & #2 are typical teen things but #3 bugs me the most. If you are having so much fun, why do you need to stop what you are doing just so you can tell everyone about it? I have never, ever been at a friend's house and felt the need post about all the fun I am having in the middle of our visit. I also wouldn't run to the Dis and post about it. I just don't get it.
As for the adults, it seems like one continuous horrible Christmas brag letter.
"My kids are perfect, my spouse is wonderful. My life couldn't possibly be any better."
The problem is, I know these people and they call me and complain about their life!
I just don't get the point. I know many people say it is a way to keep in touch but I don't want classmates from 25 years ago to see certain pictures or know that we are headed on vacation or that I want to smack my kids upside the head.

I reserve those things for special people!