* Guestbook: Probably one of the earliest forms of online discussion. A single list of replies on a page.
* Message boards: Top level thread with sequential replies, usually paginated. These get cumbersome after a few pages, and for busy threads with hundreds of pages, almost all of the content in the middle gets lost
* Facebook / Instagram: A top level post with a mess of replies that on Facebook can be sorted by relevant, on Instagram I'm not sure can even be sorted, just load more infinitely. I think these examples show really poor execution of online discussion yet are still immensely popular
* Twitter: Twitter has probably the most conversational style for non-live communication. Tweets can be replied to with tweets which can be replied to, nested conversation chains. Jumping into the middle of one of these discussion gives you context by also showing you the previous 1 or 2 tweets.
* IRC: Grandfather of chat rooms. Join a room, chat in real time, leave
* Slack + Discord: Evolving chat rooms with reactions, persistence, a nice user interface - and for Slack threaded replies which are an interesting addition - but somewhat unmanageable with many responses