Facebook knows my surgeon
reddit.com
reddit.com
1. The individual visited the doctors website, which has a Facebook tracking pixel. This pixel is associated with the doctors page.
2. Contact lists. The user has given FB access to their contacts and the doctor is there.
3. The patient used the doctors free wifi. The user browsed a website which has tracking information that leaks to a third party some identifiable information along with the IP. The third party knows that IP belongs to the doctors office, so they know you are connected. This data is sold back to facebook via one of their DMP (data management platform) partners.
I could go on. Lots of ways leak information without it being purposeful. Ad blockers also don't necessarily solve this problem as tracking can be done server side as well.
1. I have not visited the insurance agent's website.
2. He is not in my contact list. (And I've not given FB access to my contact list).
3. I've never been to his office.
I have visited the website of the national insurance company that he represents, so if he was my local agent I could see them inferring a connection. However, he is not my local agent. He's from the city I lived in two moves ago--a city I lived in before FB existed.
As far as I know, I have no friends that use him as an insurance agent. I did switch to that insurance company on the recommendation of friends, but that was three moves ago, when I lived in a completely different state. Those friends and I both moved to this state at about the same time, but to different cities, and we switched to different agents when we moved.
It was rather creepy.
Keep in mind FB is only making a guess based on attributes it's been fed. I'm sure friend suggestions may also include other people who have visited the same hospital you have nothing in common with. They're hoping to get a few friend hits by aggregating as many signals as possible.
Not using Facebook is kind of like people who refuse to use gmail... in the end it doesn't matter because every time you email someone who uses gmail, they get the conversation anyway through the other end of the link.
To avoid Facebook's surveillance network, you have to not use Facebook AND only interact ONLY with people who don't use Facebook.
Best of luck!
Escaping gmail is also possible.
Scenario #2: You are at a party and someone takes a photo of your group, and tags you in the photo. Congratulations, your name, photo, and a link to the person who posted the photo are now on FB without your consent and possibly without your knowledge. Again, you had no personal interaction with FB.
Scenario #3: You email someone at xyzcorp.com. XYZCorp uses gmail for their email hosting. Congratulations, gmail has now linked you to XYZCorp. Here also, you had no intentional interaction directly with Gmail.
The ways in which the likes of Gmail and FB build their social graph are many and varied. Don't underestimate them.
In the other thread about the psychiatrist, you can find where I discussed a similar incident that occurred some time ago between myself and a professor.
Facebook says they don't use location data for friend recommendations (anymore), and given the contact linking it seems reasonable to assume that it is correct.
I have a friend who is not on Facebook but his wife is and most of his friends are... chances FB knows a lot about him are very high.
Lesson: give facebook a throwaway email and phone number if you can, if you really do not want this to happen. Also only browse facebook in incognito mode.
The social graph has too many data points.