Interesting report from PEW last week on the methods used within social networks to maintain privacy.  There is some thought applied, and care taken, to manage the risks. 

The majority of teens actively manage their online profiles to keep the information they believe is most sensitive away from the unwanted gaze of strangers, parents and other adults. While many teens post their first name and photos on their profiles, they rarely post information on public profiles they believe would help strangers actually locate them such as their full name, home phone number or cell phone number.

Source:  PEW

There is a differentiation in the nature of information shared, and depending on the type of site.  This includes showing fake information both for fun, but also for privacy.