If you haven't consented to share your contacts with Facebook, but two people have consented who have you as a phone contact, then you're a link. This can be "interesting" when one of the parties has a duty to maintain privacy, like a psychiastrist. See this article: https://splinternews.com/facebook-recommended-that-this-psyc...
This explains a lot of the weirder suggestions I got in college. People I had done a group project with, but never communicated with outside of in-person, would show up as suggestions all the time. I'm sure in each of those cases there was one person in the group who everyone was in phone contact with.
