"I can turn you off."

Call me a Luddite, which I am, but I don't really understand cyberbullying (despite the fact that I think nobody is more despicable and cowardly than a bully, any bully).  I mean how is possible? Can't you choose what you view on the Internet? So why would you be exposed to something that is insulting to you unless you deliberately went to that site? As much as I hate and avoid Facebook, it seems to be that you chose who you wanted to be friends with and could avoid those you didn't. So why are so many young people bullied by their peers? I don't get it, and I'd like to.