I thought about all the possibilities listed and simply cannot find any of the potentials. It just seems bizarre! I cannot think of any common way FB could've connected us, other than pics.
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.
Plus, you never know how many people get their entire address book stolen by Facebook without consent (or a drive-by "can facebook app access your contacts?" button that immediately uploads your entire address book back to the mothership; or a linkedin connection/scraping or call record logging on android or half a dozen other aggregate info dump methods people aren't privy to).
I'm surprised facebook hasn't tried to create their own spyware-infused browser and computer platform yet. Millions of facebook users think "facebook" is the entire Internet anyway. Not far removed from just giving over your entire life to them.
The more I think the more bizzare it was. The reason being, we know each other by synthetic names in a forum (and let me just say anonymity was expected from both of us). So the only way I can think of is somehow the synthetic string/name matched on both of our data somehow like you are describing.
