Hacking is rare. Usually it happens if you log on from a public area like internet cafe and someone put a keylogger or something on the computer. So it can learn the passwords you use for whatever sites/and anything you visit. (Which is why you need to be cautious if you log in from a library, an internet cafe, or some other public place). But even these situations are rare xD I don't know exactly how hackers work but they are tricky. Most would not bother with a little game, they'd go after bigger things.

People just end up giving their password to a person or two and later forget. Then they wake up to either discover they cannot access their account or everything they have on their account is gone. Usually these situations are because of scammers or people someone thought was your friend. Maybe even a friend of a friend of a person who you told the password to.

Just like Akiwo said do not reveal your password to anyone.