Ask HN: Solve a Facebook 'suggested friends' riddle

[Peroni]

I've just created a Facebook account for work using my real name. I used my personal phone number and work email in order to verify the account. The only other activity I've done is to add my company as my employer. All this activity has occurred exclusively on my work Macbook. That is the absolute limit to my activity so far on facebook.

On the right hand side I now have a list of suggested friends. I know every single one of them either directly or via my wife. There is one person in particular who is a suggested friend who does some consulting work for our company but has absolutely no connection to the company on facebook.

Granted, facebook algorithms are undoubtedly a bit of a mystery but I'm intrigued (and concerned) as to how accurate its friend suggestions are. Can anyone shed some light on this?

[pbhjpbhj]

Did the list of suggested friends appear as soon as you first logged in or did it appear a time after you created the account?

I can see how this can happen without parsing addressbooks or whatever:

Someone searches for you the day you create it (perhaps someone you mentioned making a FB account to) and added you. Then you're included in to a circle of people, you get added to the "do you know" and perhaps someone else adds you as a friend, then you get suggested to other people, etc.. Maybe there are dangling tags (unresolved to a FB account) with your name in photos of the person who friended you, cross-match with the other people in the album, suggest most likely company and friendship groups from that, rinse & repeat establishing a probability for a list of people to be friends. This sounds like it would be pretty effective to me.

When you say "personal phone number" presumably you mean mobile phone? Did you install the FB app? It's not a phone number that your wife might list as an alternate is it (eg for recovering access if she loses her phone)?

[Paul_S]

Facebook can read emails and address books of your friends who might have your email address and it goes from there. Same thing happened to me. Most poeple on facebook are fine with sharing all their personal data with facebook - and that by extension includes you.

Essentially if any of your friends have emailed you or you emailed them, even once and only one of them it's trivial to rope in the rest.

[Peroni → Paul_S]

Is this common knowledge? Sounds like incredibly poor practice to sniff emails & address books without prior permission or at the very least, transparency.

[Paul_S → Peroni]

They all ask for permission and most people just tick the box. Linkedin lets you do the same thing. People give them their passwords to their email accounts willingly. Looks to me it's a pretty common practice but I'm no expert on social websites. I wouldn't do it but that doesn't matter, there's always plenty of people who will.

[trin_ → Peroni]

its a "let uns look at your contacts so we will find other friends of yours on facebook"-feature. and a lot of people (sadly) dont care about this.

[idoh]

I bet they look at the IP address as a signal - people who sign in with common IP addresses have a higher likelihood of knowing each other.

[shyn3]

I created two accounts a while ago with completely different personalities and friend circles. They didn't have any friends in common. One day I logged onto both accounts from the same PC, logged into A, logged out, logged into B.

The next day when I logged into A account B appeared as a suggested friend.

This PC wasn't blocking cookies/js or using a vpn.

[junto]

It isn't who you know, but who knows you.

Facebook is reaching a kind of herd level whereby, those people who aren't yet on Facebook, are actually on Facebook, but they just don't know it yet.