Once upon a time on Facebook, participants had to be a vetted member of a community to even have an account. Privacy was a deeply held value and many turned to Facebook because of the ways in which it protected them from making public mistakes. This was especially core to youth participation. Parents respected Facebook's attitudes towards privacy and, in a shocking moment of agreement, teens did too.
If you don't know danah boyd, you should. She looks at teen behavior in social networks. Her work is very smart and really insightful. This piece looks at Facebook's new privacy policies and how they've been changed recently. Dr. boyd (she spells her name with lower case letters) was a keynote speaker at last year's TLT Symposium ... it was a killer talk and can be seen here.