Facebook doesn't have sub-replies so I don't understand your argument. On Facebook you make a post and everyone comments to that post directly under it. On Facebook you can't comment to a comment. It just adds your new comment directly below theirs. At any rate my goal was never to copy Facebook anyway so is irrelevant.
If you need in depth replying and quoting then use the Forum integration and you'll have full Kunena features at your disposal specific to a group.
You can actually implement reply to reply by calling the reply display function inside the reply display function causing it to loop, but I have no plans to as it'll look completely awful and makes no sense. The way I understand it is you want some sort of tree like format below.
Post
- Reply
- - Reply of Reply
- - - Reply of Reply of Reply
This would go on infinitely and does not fit within GJs design or Joomlas for that matter. The only way it'd visually work is if there was force collapse of some posts and that'd require a rating system of those posts and you'd also need full page width for it to work.
Current design is as follows, which is the same usage as other social sites.
Post
- Reply
- Reply
- Reply
Maybe if you can provide a screenshot of what you're wanting would have a better idea of what you're looking for?