The structure provided by most hosts of discussions, including Facebook, is 'flat,' meaning that all replies go to the end of a given thread.
The alternative is a 'threaded' structure, which means people can insert their reply to a specific comment right below that comment.
There are pros and cons to each approach:
Flat comments are well suited to presenting broad brush-strokes and ideas that are the result of an opinion/conclusion held in advance of writing it down. This approach is the best choice when readers check in on a discussion only occasionally, or only once. It's up to writers to provide the full context when they want to delve into details. Readers, as a result, can get up to speed with less effort.
Threaded comments are well suited to digging into fine details and tracking ideas that are less fully formed. They are the best choice when readers check in (and participate in the discussion) frequently. The reader must keep track of the context for each detail. Writers, as a result, can more quickly explore a lot of different paths.
I'm interested in your views on this quick riff on comments!