On any post, you can reply in the comments but there is only 1 "level" of replies vs. being able to reply to someone's reply, and have your reply be indented from the rest. Facebook adopts a "2 level" approach to replies, meaning you can reply to the original post and then you can reply to that reply and form a deeper level but you can't reply to the reply of a reply. I believe on Reddit and Discus (internet commenting system) the commenting levels are infinite.
Which system is best to have? Please add pros and cons.