In other words, comments on articles or posts are not the only way of commenting. Think of the different ways in which a reader can express his/her thoughts on the article or blog post he/she is reading.
If I am a reader, I can
- bookmark the link using one of the social bookmarking tools and write a note as my comment
- share the link on messaging services like twitter, Jaiku or Pownce with a comment
- discuss on one of the meme sites like Reddit, Digg, Slashdot or DZone
- share the link via social networking sites like Facebook or Orkut, either as message on a wall or in a group
- discuss the topic in a forum dedicated to the topic
- write on my blog or my tumblelog, which should send the appropriate trackbacks
- directly comment on the article/post I am reading.
Of course you would ask why would I not comment directly on the post and go somewhere else. Because sometimes it can get a higher value and a higher credibility elsewhere. If the community already does not exist, I can take the comment to a place where the community already exists, like in forums or groups. Also, since bookmarking allows me to write my note on it, I can archive the link as well as my comment and contribute to the social tagging, all at the same time.
Now all this already existed from the beginning. Web is meant to be decentralized, it is supposed to provide more than one ways of doing the same thing, so that there is plenty of choice. Today direct comments hold the main conversations, but it is also true that more and more discussions are happening off the site. I have found some extremely valuable comments on del.icio.us or blinklist, or in forums on some of my thoughts. Sometimes I have discovered references by looking at referral URLs and discovered some excellent discussion on my own post.
The sheer number of the various services today make it impossible to individually track them. I do not think this is bad, because now even without comments discussions can take place. But this does mean that we do not have enough technology to track all this. We have to use a variety of means to find references, which is time consuming and tedious. Comments are now distributed, across various services and various technologies. And as a personal observation, this is on rise and it is going to get more and more difficult to track all of it.