Eventually, EVERY employer may ask this and then you simply won't have a choice

Two hours later, here is what I sincerely suggest you say:

  The people I interact with have an expectation of privacy around the things
  they share on FB with me, or even the fact that we know each other.

  When I come to work here, you will have certain expectations about my discretion
  and ability to respect the company’s need for privacy and my co-worker’s needs
  for privacy by not sharing things I see, hear, or are privy to with a third party,
  even—or especially—if I am offered a financial inducement such as an attractive
  offer of employment with a company I respect.

  I therefore ask you to recognize that I am giving my friends and family the exact
  same expectation of privacy that you can reply on from me once I join your firm.

I don’t think that’s smarmy or righteous, it’s just good old-fashioned golden rule stuff, and you are demonstrating your integrity. There will be a certain number of companies who ask to shoulder-surf your facebook, and I will guess that some of them will back down if you say those words to them and mean it.

JM2C, of course, I am not qualified to give career advice.