This is from a court deposition:

Facebook’s stonewalling has been revealing on its own, providing variations on the same theme: It has amassed so much data on so many billions of people and organized it so confusingly that full transparency is impossible on a technical level. In the March 2022 hearing, Zarashaw and Steven Elia, a software engineering manager, described Facebook as a data-processing apparatus so complex that it defies understanding from within. The hearing amounted to two high-ranking engineers at one of the most powerful and resource-flush engineering outfits in history describing their product as an unknowable machine.

The special master at times seemed in disbelief, as when he questioned the engineers over whether any documentation existed for a particular Facebook subsystem. “Someone must have a diagram that says this is where this data is stored,” he said, according to the transcript. Zarashaw responded: “We have a somewhat strange engineering culture compared to most where we don’t generate a lot of artifacts during the engineering process. Effectively the code is its own design document often.” He quickly added, “For what it’s worth, this is terrifying to me when I first joined as well.”

[…]

Facebook’s inability to comprehend its own functioning took the hearing up to the edge of the metaphysical. At one point, the court-appointed special master noted that the “Download Your Information” file provided to the suit’s plaintiffs must not have included everything the company had stored on those individuals because it appears to have no idea what it truly stores on anyone. Can it be that Facebook’s designated tool for comprehensively downloading your information might not actually download all your information? This, again, is outside the boundaries of knowledge.

“The solution to this is unfortunately exactly the work that was done to create the DYI file itself,” noted Zarashaw. “And the thing I struggle with here is in order to find gaps in what may not be in DYI file, you would by definition need to do even more work than was done to generate the DYI files in the first place.”

The systemic fogginess of Facebook’s data storage made answering even the most basic question futile. At another point, the special master asked how one could find out which systems actually contain user data that was created through machine inference.

“I don’t know,” answered Zarashaw. “It’s a rather difficult conundrum.”

I’m not surprised. These systems are so complex that no humans understand them anymore. That allows us to do things we couldn’t do otherwise, but it’s also a problem.