But completely separate from that, giving your employer your Facebook password does not give them the right to use the password. In fact, using the password to logon could be considered a crime, such as "computer fraud and abuse" or "identity theft".
Facebook's current legal terms say this:
You will not share your password, (or in the case of developers, your secret key), let anyone else access your account, or do anything else that might jeopardize the security of your account.
That means you are in violation of those terms if you give your password to your prospective employer. But, hypothetically, Facebook could add to their terms:
A account may be accessed only by its owner, by logging in you agree that you are the person who owns the account.
This makes it clear that logging into somebody else's account is identity theft, which means employers can be prosecuted under existing fraud and abuse laws. Facebook could just monitor multiple logins from a single location of unrelated accounts, and then send the police to go arrest the employer.
Of course, employers can respond by insisting that users log onto their own accounts during the interview process, but this is still an improvement. Presumably if one employer rejects you because those drunken nude party photos, you could remove them before the next job interview.