wrt above ^
I don't know if you'll see this, why not just have messaging between users, and allow references to posts in those messages.
Then either owner can just create a post, quoting the contents of the message they received (perhaps linking to poster's profile, a thought-stream, or a post, or any mix of these) at their own discretion.
i.e. they can make the private discussion public by posting its contents.
That way we don't get the runaway comments section that other sites get, since everything in a TS is still audited and maintained by its owner. comments only get in if deemed relevant, and then only in a form edited for relevance.