> How to start this? Perhaps with a confession/explanation as to my motivation for this post.
>
> I very recently sent a worried e-mail to a friend, that basically said, “Where the heck have you been? And why haven’t you answered my e-mails?” In retrospect, it was a pretty foolish thing to do. It was precipitated by a very short period of time during which this person, who is very conscientious about answering e-mails, left mine unanswered in an in-box.

There's your answer. This person is usually very conscientious about answering emails. It isn't a question of online communication, but of a divergence from normality.

If your friend regularly took several days to respond to an email, would you worry about it this time? Nope, of course not.

Likewise, if somebody who was ordinarily in the chatroom 24/7 wasn't there for a few days, I would wonder what had happened to them. If it was somebody who was there at random times, or occasionally, I wouldn't.

The only real difference between online communication and (say) writing letters or leaving phone messages for people is that it's so much closer to real time that an instant response is more often the norm, as with vocal communication in person.

To go off on a slight tangent, I hope Miss Manners never DOES have a section about online communication, because that would only add weight to the ever-growing belief that etiquette online somehow should have different standards from etiquette in ANY OTHER FREAKING CONVERSATION IN THE WORLD like not shouting at people or randomly bursting into a conversation between several complete strangers to tell them something random, or abruptly saying "this is boring goodbye" instead of leaving quietly and politely, or -deep breath- all right, I'm back to normal now.