How does Facebook know? (friend suggestions)

Factual Questions
1

So I just had two somewhat creepy facebook “do you know this person?” suggestions:

Martin <hispanic surname>. He was my gardener of about 12 years. Great guy, I’ve even met his family once when he did extended work at my place; his kids played in my backyard that day. I have had no online interactions with him, ever. We communicated by phone or text. I moved two years ago to a place with no landscape, and have not been in contact.

Leslie. Even weirder. I was single and dating in 2015. Me and Leslie communicated at first on eHarmony and then via text. Nice lady. We never met, her life was too messed up at the time. We communicated by a) eHarmony, b) texting.

Facebook has suggested both of these people as possible “do you know this person” friends. How??

Leslie I can see, if eHarmony (against all of their user agreements) shared our relationship (bastards). Martin I can’t - we have no online fingerprint together of any sort.

How how?

2

Do you use WhatsApp and did you have Martin’s number as a contact on it?

Not sure if the above article is giving accurate information, but it’s a possibility.

Do you have Martin as a Hotmail contact?

3

FB keeps their formula for deciding who you may know mostly secret so anything that can be answered here (by anyone other than a FB employee who knows) are educated guesses.

Two ways that aren’t secret because you need to give them permission are:

If you use the FB app on your phone it can access your contacts. If any of them used the same phone number or email address for their FB account they can find them that way. If you use Whatsapp it is owned by FB and they can do the same with your contacts there.

On a desktop browser if you answer ‘yes’ to allow FB to search your email contacts it can suggest them from there.

Speculation about other possible ways includes:

If they are FB friends with any of your FB friends, or friends of their friends.

If you browsed their FB profile even without contacting them.

If they browsed your FB profile even without contacting you.

If you are both using FB in a sufficiently compatible way, for example being members of enough of the same groups, liking enough of the same things, browsing enough of the same people’s profiles, etc.

4

Nope.

Nope

5

!!! Wow, I’m not cool with this. [/deletes FB phone app]

Possibly. Obviously I can’t know this.

nope

Really? huh. Again, no way to know. I’m thinking this is explains Leslie.

Nope.

6

A search will turn up numerous previous threads on this topic.

7

I’m thinking searching for “facebook” here will turn up a bunch of irrelevant threads. But not a bad point.

ETA: well, maybe not a good point either. This thread is currently the number three hit for “intitle:facebook suggestions site:boards.straightdope.com”. Hmph.

8

It’s basically just maths.

How Facebook knows who all your friends are, even better than you do

9

It’s a small world.

Look at their fb pages. Look for shared friends or interests.

10

I’m going to say they got it from your phone’s contact list.
I’m fairly new to Facebook so I can sorta track how they have added friend suggestions.
The first time I checked Facebook via the app on my phone I immediately got suggestions on people that are on only on that list.

11

ummm … i’d suggest data-mining as well as heuristic algorithms.