If Google and Facebook is ready to "show" these data, I wonder what and how much data they are hiding.

Good point. I also wonder what the motivation behind this tool is.

Furthermore, I don't understand how any of this is GDPR-compliant.

> Good point. I also wonder what the motivation behind this tool is.

Probably to tell regulators and politicians that "transparency is in our dna" we design tools to help users know who is interacting with their data

INAL but will guess it is to make facebook GDPR-compliant. They show the data they have about us, but don't know if the apps that send the data to fb have used opt-in to enable sending data to fb. Maybe time to start writing emails to the companies who have uploaded our information without consent?