Twitter is the worst example you picked for most natural structures in social networking. It’s actually a bigarse tree or a forest full of trees that appears to be a beautiful grassland when you look at it. I really hate trying to follow discussions on Twitter because it has the exact same problems you mention about Reddit and Digg, except they appear differently.
In Twitter, posts (aka tweets) have replies. These replies are posts by themselves. And they have replies (which are posts and have replies and… you get the picture). Now see this whole “tree” as a flat structure, where posts, reply posts, and their reply posts all appear at a single level. There you have it - the ridiculous but somehow immensely popular thing called “Twitter”.
To me, the right balance is always a two-level system, like that of Facebook* and Stack Exchange… and this blog: posts and replies, where replies are not posts. If you go too flat a structure, you essentially close the venue for any discussion at all (like a blog with comments turned off on all posts)**. If you go too tree a structure, you end up having the very problems of trees that you mentioned in your blog article.
A two-level system ensures discussion and also keeps it easy to follow. You make a post, and others make replies below it to discuss about it. New replies are added at the end, and you can easily follow the discussion without having to follow any tangents, branches or spin-offs. This avoids the headaches of trees while giving readers the opportunity to contribute.
- unfortunately Facebook is starting to introduce “replies to comments” in some areas of the website (I think Pages), and that’s kinda freaking me out. But still, three-levels should be manageable.
** Twitter attempts to overcome this problem by introducing a loophole called “@reply” where you reply to someone’s tweet by making your own tweet starting as “@someone”. But it gets really ugly really fast because reply tweets are LOST in a sea of other tweets… and to overcome this problem they introduced the ability to view reply tweet by expanding a tweet… Basically, Twitter is a mountain full of fixes for problems they didn’t anticipate when they thought “hey, 140 char SMS tweets!”.