> If they get a match with a high school picture for "John Smith" from a school in an affluent area, they can adjust advertisements.
Right, it's not like they gather location data and insist on real world names or buy data from brokers or use cookies. Facial recognition is the missing piece of in the puzzle!
> If your face appears in a riot, they can resell the information to the government.
It's not like governments have databases with photo identification.
If you've uploaded photos of yourself on other services and those services have agreements among themselves to share each others' (anonymized?) data, then you can be tracked across services through, among other things, your photos.
I think they want selfies to enforce a one-account-per-person policy, to limit scraping. They already enforce phone verification, but there's plenty of ways around that (e.g. services that redirect verification SMS from hacked phones, or just buy phone-verified accounts from someone else). Facial recognition raises the bar and gives them extra confidence that two accounts are unrelated even if they have similar IPs/access patterns/whatever
For this reason I decided to give up Instagram completely -- my account was flagged for whatever reason that I cannot possibly understand, and I would rather not use the service than give them my selfie
Also a great dataset for "AI" training, so your face can appear in modified form in laundered pictures.