I get random connection suggestions on FB that make me scratch my head as well sometimes, but most, if not all, are people that are 3-4 degrees removed from me. So they are friends of a friend's friend somewhere in the chain. FB makes some guesses based on how much you interact with certain people on the platform. Judging by that it makes weighted judgments of who you may know. So the more you use FB and like/share/comment with your friends, the greater your chances of seeing random friend recommendations that you may not realize there is connection to.