How on earth do they do it? ?

The other day when I got back from college u logged onto my facebook and I saw that a status had been posted by me saying one of my close friends is a ******* **** and then I went into my messages and I saw that they had sent someone a message telling them to shut up, I saw that it had been done via mobile web but I don't have credit or free Internet & it was about an hour ago when I saw it but I'd been at college at that time, so I'd obviously been hacked so I changed my pword and security question on hotmail AND facebook. I was just wandering how someone managed to find out my pword and email address when I don't give out my details to anyone and I always log out after i've finished so how on earth did someone manage to hack me does anyone know?