Can you get in legal trouble for accessing someone else's facebook account?

Me and my friend were having a debate as to whether you can get in trouble for accessing someone else's facebook without using hacking methods (e.g using social engineering or simply guessing it) and if no malicious activity takes place. I always thought that the illegality came from the methods used to get the password and what you do while having access to the account. He says that if you log in, using any means at all, it is illegal. Can anyone help settle this?