See <http://www.us-cert.gov/cas/tips/ST06-003.html>
Staying Safe on Social Network Sites
The popularity of social networking sites continues to increase, especially
among teenagers and young adults. The nature of these sites introduces
security risks, so you should take certain precautions.
Excerpts:
How can you protect yourself?

* Limit the amount of personal information you post - Do not post
information that would make you vulnerable, such as your address or
information about your schedule or routine. If your connections post
information about you, make sure the combined information is not more
than you would be comfortable with strangers knowing. Also be
considerate when posting information, including photos, about your
connections.
* Remember that the internet is a public resource - Only post information
you are comfortable with anyone seeing.