Instant messaging systems tend to facilitate connections between specified known users (often using a contact list also known as a "buddy list" or "friend list").
Depending on the IM protocol, the technical architecture can be peer-to-peer (direct point-to-point transmission) or client-server (an Instant message service center retransmits messages from the sender to the communication device).
The Zephyr Notification Service (still in use at some institutions) was invented at MIT's Project Athena in the 1980s to allow service providers to locate and send messages to users.
Parallel to instant messaging were early online chat facilities, the earliest of which was Talkomatic (1973) on the PLATO system.
Initially, some of these systems were used as notification systems for services like printing, but quickly were used to facilitate communication with other users logged into the same machine.