History of Instant Messaging
Although the term "Instant Messaging" didn't come into use until the 1990's, it has been in use since the mid-1960's.
It first started to appear on operating systems like CTSS (Compatible Time-Sharing System) and Multics (Multiplexed Information and Computing Service. These systems began as notification systems but quickly started to be used to communicate between users who were logged into the same machine. Because there were only large networks, many users were in the system at the same time and working on the same machines and/or system. As networks began to develop, protocols were created and peer-to-peer communication was begun. Other protocols required that peers had to be connected to a specific server rather than connected on the same machine.
The Zephyr Notification Service was invented at MIT (Project Athena) in the 1980's so that service providers could locate users and send messages to them. This system is still in place in a few institutions.
Bulletin Boards began to be used in the 1980's and this the beginning of what we think of as instant messaging although it still had a long way to go before it became recognizable as the system we know today. AOL was probably the most well known of these bulletin boards although CompuServe was the first. I had a CompuServe account and then moved into America Online when it debuted in the late 1980's.
Modern Internet based services began in the 1990's with an early pioneer ICQ. ICQ was eventually purchased by AOL.
In 2000, an open source product called Jabber was introduced. Jabber standardized the protocol using the name Extensible Messaging and Present Protocol (XMPP). This protocol allowed XMPP servers to act as gateways to other services opening the door to what we consider to be Instant Messaging. Now there are a number of IM services available many offer video calling features, VOIP, and Web Conferencing. Web Conferencing can also include both video calls and IM. Some offer desktop sharing.
It will be interesting to see where IM goes in the future.