From internal leaks within the company as well as external analysis, the tip of the iceberg behind Facebook's spyware empire is exposed. Take a look to better protect yourself when you're not even on the platform: https://simplifiedprivacy.com/facebooks-corrupt-off-platform-surveillance/
The therapist / client thing is so true. Within the last year I had an old therapist from like seven years ago show up in “people you may know” suggestions and one of the most annoying things is they don’t tell you why they think you know each other. Proximity? Is that person uploading contacts? Who knows.
Did you ever use their WiFi? Joining on historical IP addresses would be easy.