Here's a comment I posted on the blog of my colleague, Henny Swan, in response to her post entitled Did Twitter kill commenting?:
Some additional thoughts (expanding on the excellent points above):
When you comment on a blog, nobody but the blog author, random visitors to the blog, and yourself knows that you commented. I guess I could comment here and Tweet that I did so, but that's two steps instead of one (for both me and my Twitter followers).
It's also hard for me to keep track of all the different blogs where I've commented online. On Twitter, at least, I can look through my own update history to see the things I've been interested in and writing about. Similarly, most web forums retain a full posting history of each user.
We want our impressions to matter; for many of us, that means we it to be part of our recorded history. Others go farther, and want that recorded history to be public. Unfortunately for them, blog comments are easily forgotten and difficult to recover once memory fades.
Blog commenting doesn't usually allow one to keep track of responses. After I respond here, I won't know if you or anyone else responded to this comment unless I make an effort to check back here (forever)? Some blogs allow you to subscribe to comment feeds (via RSS), but that's potentially a lot of feeds to keep track of, and a lot of irrelevant comments to wade through. Mail notification when new comments are posted also results in a lot of unwanted messages, especially if the topic generates a lot of comments.
Finally, I noticed that Twitter produces an interesting curiosity effect. People link to content (often obscured by tinyURL and other such services), and people get curious about it (because of the mystery). The effect is even more pronounced when people publically reply to each other on Twitter, and they're saying things that only make sense in the context of their own conversation, so third party observers find themselves following the Reply trail to see what they're talking about.
By removing context clues, it's like Twitter added/maintained a level of inconvenience/opacity to encourage users to be curious about other people's conversations and have to dig a few levels before getting a payoff (generating a gambling-type of thrill, perhaps).
Following Miquel's lead: twit back on @lawmune
This topic is also related to the alleged death of blogging.