The problem is Facebook data is on a lot of other companies' servers at this point, and nobody knows who has what data.

I fail to see why determining where it lives is anyone's problem other than Facebook's, and why they should not be held culpable for it.

I'm not making a moral point about blame and culpability here. I'm making the practical point that it's impossible to delete all of the data if nobody knows who has it.

Yeah it's really going to suck for FB (boo hoo), they just can't win. It's years past due. The warnings have been on the horizon and they've only amped up their data collection to the point it's become even more unmanageable than it already was.