When people say that their Facebook or Myspace "was hacked into"... what is the usual cause that led to the successful hacking?

So many people claim that their favorite website accounts were “Hacked”. What is the most common mistake these people made, if any, that led to the hacking?

I am already aware of how to hack into an account using Security Question answers (the Sarah Palin method).

But what are other ways people hack into these accounts? And can this be prevented?