When giving away your data to Social Networking sites. You reveal yourself to a third party(the social networking site provider) even when your profile is marked private. As creators and maintainers of the site have whatever access they like to your personal data (which you provided).

Keep in mind that everything that has gone public can be used against you. Whether it is using your e-mail address to deliver spam to you or leaking personal informations about you to somebody you do not want to have knowledge of this sensible data.

relevant external Links

http://en.wikipedia.org/wiki/Social_networking#Privacy_issues

http://en.wikipedia.org/wiki/Use_of_social_network_websites_in_investigations