There is a difference between what is known and what is knowable. For example if you asked me now if I know where all the receipts are for my tax-deductible do

There is a difference between what is known and what is knowable. For example if you asked me now if I know where all the receipts are for my tax-deductible donations, I would say no, I don’t know. Why? There is no reason for me to know right now. But if the IRS told me I was being audited and must produce those records or go to jail, I would find them. And I have confidence that I could, because I know that in general, those are the sorts of records I keep (somewhere). Most Facebook employees do not need to know “where personal data is” to operate their business, so they don’t know that. But at the same time, Facebook could not operate if the data was “nowhere” (destroyed), or if it was randomly distributed with no queryable structure. So the question is not really what they know now, it is: what incentive do they have to go find out.