While it's easy to be private in public offline, doing so online can be quite difficult and frustrating. Teens, in particular, complain about those who hold power over them thinking that they have the right to look just because it is possible to be seen. By this logic, anyone who can get access to someone's diary has the right to look and anyone who has the ability to overhear something has the right to hear. Just because it's possible doesn't mean it's socially appropriate. What's at stake is often a question of trust. And all too often, parents erode their relationship with their children because they believe they have the right to snoop.

Danah Boyd [the original Social Media Expert :D] keynoting at the recent SXSW. The whole talk on privacy is worth reading.