I have frequently had a business associate with whom I (at best) have a passing acquaintance recommended to me. We have no FB friends in common, live 10 states away from each other, are not connected via Linkedin or some other social website. In fact, I have only met the man on two occasions in person, and shared maybe 10 phone calls over 13 years.
How could they possibly make that recommendation?
If you or the other person has the facebook app on your mobile phone, facebook downloaded your contacts from your phone. Facebook then recommends friending people in your contacts who have facebook accounts.