A lot of people these days write short tit-bits on social networking sites instead of paragraphs and journals of hand-written notes. I think it's an evolution of the Orkut scrapbook and the Facebook wall - you would now post messages to your own scrapbook/wall and people following your mini-blog get notified, often via a free SMS text message.
When you let visitors add comments to your posts, you would reply back to their comment and there's no way for them to know that you've replied to their comment. Blogger.com addresses that by emailing you all comments after yours (or at least that's what I think it does) if you check a little box when posting a comment.
Some systems are smarter and maintain a tree-like structure to keep track of which post is a reply to which other post, but then it all depends on how the designers intended it to be. Being an architect myself, I have to make trade-offs between cost and functionality and, in the end, the more you spend, the better stuff you can come up with.