Everything you ever put on the net is kept. Even if you think you have deleted it...you haven't. Multiple servers cache the net from the company who own the pages you post on to companies that run archive services like the way back machine where you can type in a website address and see it as it was when it was first built, try it
http://archive.org/web/web.php http://archive.org/web/web.php .
A lot of stuff is not accessible by normal search engines and is often referred to as the deep web, learn more about that here
http://en.wikipedia.org/wiki/Deep_Web.
I try to tell this to my teenage daughter, the future is going to be so different, anything they type into the net can and will be used for/against them one day, maybe in a job interview for example!
I already look at Facebook before I interview someone and in America interviewees have been asked for Facebook logins and passwords so employers can check them out! There is even a bill going through to make that illegal.
The futures frightening really...everything you type can be read by someone who may have an influence in your life and I for one welcome our new overlord masters...*
* Simpsons quote if you don't get it.