When people go to events, they take pictures. They post them to Facebook for the whole world to see. This picture was taken by Ziko and everyone in the picture can be identified by the people who know them.
There is effective software that makes an educated guess based on previous identifications. I wonder how effective it would be on historic pictures. At Commons we have many pictures without the names of the people in the portrait.
These three young ladies are known to be the daughters of the Sultan of Jogjakarta. It is likely that there are other pictures of his court with these young ladies. When they are identified in one picture, it may be possible to identify them in this picture as well.
A lot of money has been invested in such spying software by many organisations. It would be cool if the resulting technology was put to use for something uncontroversial.