They're not just for spotty adolescents!
Social networking sites allow users to create a profile of themselves and then get in contact with other people who have also created a profile, usually friends, work colleagues and family, but also complete strangers.
You can search for people on these sites and, depending upon the site and the level of content that the person makes publicly available, find out about them, their work, their friends, and so on. A bit hit and miss, but can sometimes be useful.
Here are a few of the more well known networking sites:
Popular British site which has a search facility which allows you to search for people's profiles. Also has sites in other countries which are listed on www.friendsreunited.com/
Another well known site that also has a search facility which allows you to search for people's profiles.
Popular with schoolchildren especially, although it is trying to expand to encourage adults to join. Does not have a search facility, but you can search the site using Google.
I.e. [ search term site:www.bebo.com ]
A very fast growing site. Does not have a search facility, but again you can search the site using Google.
I.e. [ search term site:www.facebook.com ]
A much more business oriented networking site than the others, which allows you to search for people's profiles on the site. May become a very useful research tool in the future, if more people are persuaded to use it.
A list of social networking sites can be found on Wikipedia.