When the internet is supposed to be facilitating communication between human beings, I have found it annoying at times, especially on some IM systems.
It's simply irresponsible to halt a conversation leaving an impression or even explicit statement that it'd be resumed after a moment or two. However in fact, one party has to wait in vain.
What is more boiling, is that when one doesn't reply to a message. I understand that sometimes one is not at one's desk but it'd be nice to get back to the other party when one is back. I always apologize if I don't get back to someone on time. I think it's just a little polite gesture that is required on another being - respect and not being made like I am nobody.