Hacking Facebook account how?

[Barbie]

Hi
So I was just on facebook as usually. But suddenly my friend wrote to me and said he was on my facebook. I didn't believe it because it isn't possible to hack a fb account or is it? So I said to him that he should write something on my wall on my account.
And he did!! He doesn't know my e-mail or password or anything. How can that be possible? And if it is that easy to hack account, then why do hackers dont hack famous people? Im just wondering.
I'm not going to hack but I'm very curious about these kind of things. How can a person hack an account. Did he downloaded something or what.?

I hope someone can give me an answer
I havent been on that account on pc so I dont think it can be keylogger. I've only been on that account on my phone. I dont understand it
It's a friend from the Internet. I dont know him in real life, so he doesnt know anything about me. But I think that he used a program. I've heard that you can hack by using a program. It cant be a keylogger I havent clicked on links or something. He hasnt only been on my account but also on his other friends accounts.