[mortehu]

> Do you gather data about those "unknown" persons

This part is easy to answer: if a user uploads a photo of you and there are other people in the photo or a recognizable location, then that's data about you. Even if it's just stored as a photo now, it'll take a neural network a millisecond to turn that into feature vectors that can be used to build a knowledge graph. This goes for Dropbox as well as Facebook.