No problem with that. And I can say no, and thank you for the opportunity of the interview.
Or... you can give it to them. Then go home that night and proxy to your account and harass yourself. Then go to work the next morning LIVID. Boss will be liable for the security, no? "I was the ONLY one who knew my password and now I'm getting all kinds of crud on my Facebook page and the only thing that changed is other people know my password now, one of them is my Boss, so the finger gets pointed at him" kinda thing.
Or... just say nope. Maybe warn them... "Do you really want to be liable for my security?"