I don't think this is much better. I wouldn't work for someone who asked me this question, even if this were the rationale. Who knows why someone would divulge their password. I bet there are reasons other than being completely devoid of common sense.

Regardless, I bet that any predictive relationship between willingness to divulge a Facebook password in an interview and the ability to perform adequately as a software engineer is weak at best. To me, an employer who doesn't understand that shows poor capacity for unbiased reasoning.