An overview of technology patents for which Facebook has applied, with many imaginative ways for the company to add to your dossier and fill in details of your social graph:
“Facebook Knows How to Track You Using the Dust on Your Camera Lens”
Kashmir Hill and Surya Mattu, Gizmodo, January 11, 2018
Facebook claims that they aren't currently using this tactic. Uh huh.
One filed in 2015 describes a technique that would connect two people through the camera metadata associated with the photos they uploaded. It might assume two people knew each other if the images they uploaded looked like they were titled in the same series of photos — IMG_4605739.jpg and IMG_4605742.jpg, for example — or if lens scratches or dust were detectable in the same spots on the photos, revealing the photos were taken by the camera.
It would result in all the people you've sent photos to, who then uploaded them to Facebook, showing up in one another's “People You May Know.” It's be a great way to meet the other people who hired your wedding photographer.