https://gizmodo.com/how-facebook-outs-sex-workers-1818861596
From what I have seen shared location, being on the same Wifi and letting facebook access contacts are the big issues.
It's s shame that this issue focuses on sex workers rather than other "more socially acceptable" professions: counsellors, therapists, doctors, lawyers and the like. It's a big issue that gets ignored because people think that privacy is only for people with something to hide.
A followup and previous article from Kashmir Hill on this topic:
https://gizmodo.com/people-at-facebook-don-t-know-how-facebook-works-1819408514
https://gizmodo.com/facebook-figured-out-my-family-secrets-and-it-wont-tel-1797696163
None of this makes me sad that I closed my Facebook account