FB suggests friends after visiting same city, don't use FB phone. How?

You are missing the most obvious and most likely reason for matching with new coworkers. Your new coworkers searched and viewed your profile on Facebook after meeting you. This is a key way Facebook suggests friends.

It could be WhatsApp but I think it would be trivial to figure this out any number of ways.

You have a Facebook account, if you don’t check it on your phone presumably you do on your notebook or desktop. You probably browse the internet at home from the same WiFi network with both your computer and phone. Different cookies from the same IP address would indicate the same household. It wouldn’t take a lot more to know they belong to the same person. IP addresses can strongly correlate with a geographic area.

Since nearly every website has those social media buttons Facebook can track you from site to site even if you’re not logged in.

For sure Facebook knows where you live and has matched up your facebook-free phone to your Facebook account.

I’ve got no experience with actually doing any of this, it’s just what seems obvious to me. I’m sure the whole truth of it is much scarier.

I haven’t had a Facebook account in over a decade but my current girlfriend had my ex-girlfriend from 15 years ago pop up in her recommended list. We live about 10,000 km apart.

If there are any Facebook engineers reading this I must salute you. Your work is amazing.

I used to date girls on Tinder with at least a few degrees of separation (ie, no friends in common), didn’t have Tinder and Facebook connected in any way, and the dates would still show in my friend recommendations list.

It was WhatsApp.

My guess is phone number matching. Your friends use Facebook on their phone and added your contact information recently. That’s enough for Facebook to figure out that it’s a potential match, given that you already have some friends in common.

Is it possible your friends searched you up?

Did you add each others' numbers on WhatsApp?

Or if you moved due to work and you got added to a company-wide address book which is also synced to employees' phones and in turn got synced to WhatsApp, Facebook's systems could probably match up names to suggest a connection.

Although, why only these 2 people and not others from the company? Scarily there's probably enough data between you 3 for FB to know that you would be compatible with each other, so maybe Facebook figured the same thing you 3 figured out in real life -- you hung out with each other so I would deduce you found these 2 co-workers interesting. But I doubt anyone got paid to make such sophisticated stuff, there's probably a simpler logic that offers a quantity but not quality of "friends".

I still want to write a short story that starts with a guy saying "OK Google, get me a girlfriend", and an algorithm that slowly pops up suggesting events and locations for him to visit, knowing that a girl that he'd be compatible with will be at the events/locations...

You visit a website and if they have fb pixels/tracking, it is very easy. FB is everywhere, EVERYWHERE.

Download your FB information and scan through it. If you can, ask your colleagues to do the same. FB tracks the websites you are visiting so the local comedy club website is a possibility.

If I create a brand new Facebook profile, and I never tell Facebook my location (neither via my browser geolocation or profile info), however most of my Facebook friends that I chat with daily live in a town called Hackerville. I've never installed or used Instagram or Whatsapp.

Guess where Facebook thinks I'm living?

They can be pretty sure I either grow up or live in Hackerville.

FB could buy your data from other networks, provided via some other app.

Also your coworkers probably looked at your FB profile.

I think even your cell phone location data by IMEI is available for sale these days.

It’s really impossible to know without more info, as people have mentioned you can download your fb info for some clues. One possibility I haven’t seen mentioned is they are using your credit card data that they’ve purchased.

Either WhatsApp or Instagram.. all FB properties share a common user graph. Or your friends searched for you on FB.

I suspect it's through a Bluetooth beacon. Basically analytics/trackers for physical stores.

Sounds like WhatsApp and/or your phone number connecting the dots.

Local comedy club plus shared friends.

Voice recognition, cameras always on and recognizing, could be a million things.

My guess is WiFi-triangulation by WA. Just a guess, though.