What I mean by continuity is the core of how replies and threads are handled. There's a bazillion features you can add that will affect how conversation plays out, of course. Take the examples of YouTube and Slack, I say they're both hybrid, but they're clearly very different. Slack is a messaging application where users can create and join workspaces, hubs that are then divided into rooms: channels and private conversations. Messages are displayed chronologically and can be "reacted to", adding an emoji beneath it, though these do grant a post priority, it's still all displayed chronologically. YouTube is a video hosting website, where a user can create a channel and upload a video, and this video will have a comments section attached beneath. Comments are shown not chronologically but instead have different priorities depending on age, number of replies and likes. And there's loads of other differences. But the key common feature is that in both of their respective spaces there is a set of primary replies which each can become a sub-thread, but cannot branch further than that. That's the core of the thread form, and that alone does not mean the two platforms' conversations are going to be alike.
So yes, features do affect the development of conversation, but those features are not the form of the thread. I'm being, like, autistically specific.
On Twitter, I do not believe pinging or links make it a fourth form. Notifying another user of the post made is not the same as replying to multiple posts, it's still appended to a single previous post. Nor do cross-links change the form of threads, I can link >>2301
, but we both know this is one thread and that's another one, you may use the link to visit the other thread but they remain two separate entities. What could be a fourth form is that format based exclusively on tags I've heard about a couple of times, without channels, threads or direct replies. But I don't know anything about it.