Keep in mind that with other communication mechanisms (e.g. email, SMS) we already send over a copy of the message and keep the original. I'm not saying it's "better" from a privacy perspective, just that it seems like the logical solution here, and I'm not sure how a court might conclude otherwise. The data is being hosted in Europe at that point. It's just that a copy needs to be sent to the recipient only when the message is initially sent (because how else do you communicate?!).

That's true for some kinds of PII data and not others. The social graph (who are your friends) is symmetrical. Shared-edit documents, dropbox-like file sharing, and wikis are often ownership-ambiguous.