Well I think it’s now become apparent that Facebook has been a wonderfully successful attempt at consensual data mining. Chris, you called it. I’ll say that here and now. It’s also made apparent the decision I’ve mused over.
I’d also like to point out that while the argument is based around the idea of a conversation having two end-points and each having a copy, the idea of the middle having a copy has classically been referred to as: “the Man in the Middle“. Would you like the mail/postman photo-copying all your mail ?