So with the latest NSA scandal, probably the last one using a computer has become aware that everything you say and do on the internet can be tracked in one way or another. What it comes down to is very simple; the only place that remains truly private is inside our heads!
Anything you type into a machine, whether on a social network or even a "private" email etc., is in the end accessible to at least the company whose service you are using to write an email. And they do access all your emails. As an example, wrote to a friend about Poi once, and the next day I had advertisement linking me to Poi resellers etc....
So: privacy and the internet? Simple: you want to keep something private, don't put it on the internet in any way shape or form. If you put it out there, it is accessible.