Insofar far as I understand GDPR, models (2) and (4) can run anywhere, provided that the data are ephemeral, encrypted in transit, and their output decision can

Insofar far as I understand GDPR, models (2) and (4) can run anywhere, provided that the data are ephemeral, encrypted in transit, and their output decision cannot be queried later (e.g. by logging a relationship between the input to the model and the eventual message delivery that includes PII like IP). The question of "where is the discussion stored?" would indeed be problematic if it were not possible for it to be encrypted in such a way that a key from both countries was needed in order to reveal the conversation plaintext, but I don't see a clear reason why that should not be possible (it may not be how Facebook actually stores conversations now, of course, but it is not a technical barrier). As far as I can tell, the really difficult aspect here is how and where to permanently store the fact that the two users are talking to each other once the comments are actually made, since the mere fact that they are talking to each other demonstrates a relationship between them which may be considered PII in some contexts. Or at least, it would be difficult if the US also had privacy laws like the EU's, and IMO any coherent solution should be able to work if the US adopts something like GDPR. Unlike the message contents, this is quite difficult to store in a privacy-preserving way. I think the discussion would be more interesting and feel less like attacking a strawman if people were to focus on the interesting questions like this one, rather than the extremely uninteresting question of whether Facebook can serve posts to the US at all (which it obviously can).