Guess it depends on how you define it; the name comes from "Bulletin-Board System" which describes the way "Talk", the program I used to use on the PDP-11 VAX from 1976 on, worked; multiple users could post to various subjects, visible to all users, just like on a bulletin board, etc. We wrote our own Email programs (the sysop considered such things to be games; "Electronic mail will never have any practical use") and our own real-time instant-messaging program (called "Phone"), all of which we used long before 1980.