Many schools, parent groups, and even Stanford are offering classes or informational evenings about Facebook to parents and I encourage everyone to make the time to attend one. Not only will you feel better about your child spending time on the Internet, but it will increase your "cool" factor if your eyes do not glaze over whenever your teens talk about being a top girl or writing on a wall.
Parents are responsible to know what their teens are up to and parents who are involved with their children have better relationships with those teens and teens who are involved with less "risky behavior" e.g., everything you do NOT want them doing. So go ahead, take the social networking plunge!
If you have a profile on Facebook, you will be able to be a "friend" with your child and see what is going on in parts of their life they may not share verbally. It is also a great way to stay connected to your teen once they go to college.
One benefit I had not anticipated is that it makes "connecting" with people from your past much simpler. People from your past who are much more facile with this "new world" will find you, even if you do not find the time to reach out yourself.
To find out more, you can go to:
Facebook for parents - a site by psychologist BJ Fogg for parents that will walk you through starting a profile, friending your teen, reviewing his or her profile, seeing who is friends with your child, and using Facebook to create your own social network.