I would just tell them I don't know it, and store it in my password database at home.
Come to think of it, I would not even be lying if I said that. I really have no idea what my facebook password is. I have a pins file with all my passwords, and it's in there. If they wanted me to email it, I'd just email them the wrong one, and give them lot of hassle. "Oh I really don't know why it's not working, it works fine for me at home".
But it's getting pretty bad when employers are starting to ask for this stuff. Must be a US thing for companies/government to like invading people's privacy, as this stuff does not seem to happen here, at least not to this extent.