Connections like these seem inexplicable if you assume Facebook only knows what you've told it about yourself. They're less mysterious if you know about the other file Facebook keeps on you — one that you can't see or control.
Behind the Facebook profile you've built for yourself is another one, a shadow profile, built from the inboxes and smartphones of other Facebook users. Contact information you've never given the network gets associated with your account, making it easier for Facebook to more completely map your social connections.