I have had an account on Facebook for almost 9 months. I use it to communicate and share photos/information with my family and friends, many of whom live all over the world. It's free and easy.
I prefer Facebook to other social networking sites as I can choose who I want to be friends with. I like that people who are not in my Friends list cannot see my full profile and posted photos. I'm more of a private person and don't like to have too much personal information on cyberspace for everyone to see. I also don't like the idea of strangers looking at photos of my daughter.
Since the 9 months that I have been on Facebook, I have been contacted by friends from high school whom I haven't seen or talked to in ages. It's been very fun reconnecting with them! I currently belong to 3 groups but don't want to add additional groups to my account at this stage. I have a hard time as is with work and raising a family to find time to constantly check my account.
I can definitely see the appeal of social networking sites. I am also not surprised that teenagers make up a bulk of the groups. There is a certain freedom on the web where you can be who you want to be. I heard a very interesting piece on MPR about how parents are warning their kids not to put too much personal or inappropriate information on these sites as it can come back to haunt them in the future e.g. when you are looking for a job after you graduate.
I, for one, am happy that my daughter is only 2 and does not visit these sites yet.