Well - maybe - but since 36 million people can already see you INSIDE facebook, the other 4 billion is a bit irrelevent isn't it?
It's a brave new world... I can see you getting into your car from space and I can see your name on the SERPS. But I USED to be able to see your phone number in the phone book AND your address, so I guess not so much change after all.
In honesty - I think you are right. This IS a mistake for Facebook and somewhat immoral not to have it "opt in" instead of "opt out". I am especially surprised that the defaul (as I found out yesteday) allowed searchers to send an email through the facebook system. Although presumably using a Catcha to prevent bots.