Its not called HACKED when you use a public library computer to update your Facebook and you forget to log out. Then the next person to use the computer takes the liberty to play around with your profile and wall. Its called being stupid and careless.
If Facebook ever shut down, you'd see people aimlessly walking round streets, scribbling on walls, poking each other, searching for their friends, thumbs-upping and commenting at everything they see and tagging one another.