Back when IRC first came around it allowed people to contact another person in real-time and sometime later, Instant Messaging such as MSN, GTalk and others were born. Over time these IM clients involved running on just about anything that has a connection to the Internet and thus with this people started to get used to being able to contact anyone at any time and expect a reply back right away.
I am sure everyone knows that person who will always keep sending messages when you do not reply in less than a microsecond. I am sure you also know the ones that if you do not reply in a nanosecond they say: