I know this is /. and everything typically has to be boolean/polar, but how about some more processing before rendering a knee-jerk yes|no and running to the high ground of your position ...
Ask the interviewer "Can you tell me what reason you need to see a personal account of mine such as Facebook?" If you're on track to a high-profile position, support of one or one where security is paramount, they may have a reason. I mean ... I know no politicians or folks in the public sector have done inappropriate things such as maintained inappropriate relationships or done shady business using just such accounts, but hey ... it just might happen someday, right!?!?!? So ... they may have a good reason to ask from their side. Some jobs do require background checks. This could be filed under that. That doesn't mean you have to give it to them. It just means that they have a [potentially valid] justification for it. If it's a wal-mart greeter position, I go report them to corporate and/or file a lawsuit. If it's part of the foreign service officer application process with the state department.
Ask/point out that you are uncomfortable with exposing friends/families information (as well as your own). Again, a security/background check may trump that anyway (if it's a condition of the job). While it's a policy, the human in front of you may actually consider that point.
Ask "How do I know what I show you will be kept confidential?" ... "Is any of this recorded digitally?" ... "May I ask how this factors into your selection process?" ... maybe even without being argumentative.
Maybe even ask them ... "Don't you wish you could forget all the inane* conversations/posts/etc. that you've seen doing this?"
Then ... if you don't get the job, ask "Can you tell me why I didn't get the position". If you feel it was related to one of your (or your friends') inane posts on facebook and/or it's discriminatory (e.g. they didn't hire you because they saw photos of you with blond hair on your facebook timeline and they don't like people who dye their hair) ... go for your lawyer ... or move on to a different job interview.
*Because yes, there is a whole heck of a lot of inane stuff on Facebook.