Facebook started giving me friend suggestions of people who worked there. Location wasn’t even enabled on my phone.

Some devices will still passively check SSIDs, even with WiFi "off", then they just look for users who have similar nearby wireless networks.

Bluetooth? Maybe even ultrasound to detect nearby devices?

I don't think you can have privacy online anymore. The best thing you can do is make a lot of noise, so it's hard to determine what data is real, and who it belongs to.