From what I understand, the case by now is about "Is what companies use to justify processing of data in the US as safe acceptable?". The ECJ might very well find that the current agreements are not sufficient, that data transfer to the US likely means access by US agencies that is not acceptable, so Facebook has an interest in there not being a decision by the highest court about it.
I wonder if we're heading to a world where any company who wants to run internationally needs to keep all US-citizen data on US servers, and EU-citizen data on EU servers?
Currently it doesn't look like holding US-citizen data on EU servers will be a problem anytime soon. But if the EU decides that the protections for their citizens provided by the "privacy-shield" agreement are not good enough then it would mean US companies could not hold or control EU-resident's data.
