The other side probably got notified too. I remember seeing a peculiar name on FB's "you might know..." list, I wondered where I saw that name before. I looked in my mail inbox, and it's a guy who sold me something on eBay a few years ago. I never gave Facebook access to my mail account, but what I'm pretty sure happened is that guy did just that. FB saw that we exchanged some emails, and said (probably to both parties): "Hey, be FB-friends with this person!".
I wonder what the security implications are. I think a security researcher added a lot of phone numbers into his phone and let Facebook read it, and Facebook responded with names and faces of those numbers.
This happened to me too. There was a guy on Reddit looking for help setting up Plex on a dedicated server. I run a small chat room and helped him out in there.
But we did exchange a few emails. I use fastmail and he uses gmail. A few days later he pops in the people you might know thing. At the time I did not have a cell phone so I am assuming he gave the app access to his contacts or something.
I think it is safe to say Facebook has a pretty detailed profile about you even if you never made an account.
Facebook can't even guess my name on emails I receive weekly about what my “friends” are up to. They think my name is Martin Salum.
ps: never used facebook.
As unnerving as it might seem, let's not forget that Facebook has a HUGE database of faces and relationship with those faces.
Even though is not active, Facebook AI could easily auto-tag me in every photo ever posted by anyone, friend or foe.
Therefore, facebook could also make a connection between your "real me" and your "other secret me", purely on a set of photos, tags and profile pictures
RE the phone number thing, isn't that common knowledge? You can type in any phone number, and if their privacy settings allow it, their profile pops up.
I wonder what the security implications are. I think a security researcher added a lot of phone numbers into his phone and let Facebook read it, and Facebook responded with names and faces of those numbers.