It builds a detailed profile of you based on your public data: https://news.ycombinator.com/item?id=22681207

A startup: Face classification and person tracking via photo scraping social media, as a service, offered to law enforcement agencies, and they keep getting hacked.

If they keep scraping does that mean I have to keep opting out?

Correct me if I'm wrong, but if you opt-out, they agree to stop adding to stuff they can identify as yours.

That doesn't stop another company from doing the same. And that's the problem. There are so many companies that we don't know about.