Reading email is fun, but responding to email is work. Each new email that appears in one’s inbox carries with it conflicting feelings of excitement (breaking news!) and dread (obligation to respond).
Other point-to-point communications, such as text messages, have a similar dynamic. I’ve heard a few people say that a benefit of Twitter over texting is that there's no obligation to respond.
Jay Deragon goes into more detail, defining obligation and how it affects new communications technology. He quotes Deborah Schultz:
In an era of increased inputs, ambient intimacy and weak ties, it is now difficult to remember, react and respond to everything and everyone in a timely manner. Are we rude or do we need to reset our expectations of what is proper etiquette in the era of the Social Web?
“Ambient intimacy” is a good term to describe how Twitter, Flickr, blogs, and other modern communications technologies keep us in touch with one another. The term I’ve been using for this is “passive communication.”
Whatever you call it, keeping in touch through passive means allows us to stay in closer contact with a greater number of people. We can escalate that contact to something active (via a comment,@reply) whenever we feel like it, but there’s no obligation to do so.