Did you join based on an "invitation" from a friend or did you go directly to the site and join?
I know I have been hit with MANY invitations to LinkedIn and I ignore all of them. Many have come from fellow Cruisemates.
Social Networking sites (Guess what? Cruisemates is one) can be great but you have to pay attention to what you are doing and what you see from others.
I am the PIA on Facebook that will post hoax alerts, tell people if what they post is a hoax and inform "friends" that they are making "Global" postings that everyone can see.
I love Facebook because it helps keep me in touch with my family all over the world and have the ability to keep up. Heck, it opened the door for me to discover my family in Australia. We met up with them in England, last year and met with another group in Las Vegas this year. Next year we will be invading them and they are thrilled.
However, I am very picky about who I "friend". If I don't personally know them then I don't accept the friend request. I also set up "Groups" so I can post things to only certain people, like Family, close friends, etc..
Social media is here and it will stay. Like everything, you just have to take some time and common sense to learn how to properly use it.