warpus
In pork I trust
If your FB account is private though, and an interview wanted to see it to make sure there wasn't any objectionable content, or to make sure you knew how to use FB, they could ask you to log in in front of them and review the content (which is what I imagine the person in the OP's scenario wanted to do), without violating FB's terms.
If the job would involve you using your personal facebook account, and that ends up being tied to the company on a day to day basis, then I see that as the only legitimate reason for somebody asking you to log into your facebook account and show that the content is not objectionable in some way.
But how many jobs like that exist? Your initial OP seems to ask the question in a much much broader context.