Exactly. Heck, their available profiles for iAds shows that they could personally associate everything from location to financial status to all our past history of iTunes usage.
Indeed, almost all my own teen daughter's friends have iPhones simply because of Facetime and iMessage.
It is very enlightening, however, to read all the comments about the most popular messaging apps in the rest of the world!
The problem with differential privacy is that the more it obscures individual inputs, the less useful to an individual it is. Unless Apple figures out a way to re-combine data stored on the device itself. That would be cool, although tied to a single device. (I prefer server side storage of my preferences, so they work for me on any device or browser, anywhere.)
Which is why this comment doesn't make sense:
"When I asked a senior Apple executive why iMessage wasn't being expanded to other platforms, he gave two answers. First, he said, Apple considers its own user base of 1 billion active devices to provide a large enough data set for any possible AI learning the company is working on." - Mossberg