Use of electronic media even predates messaging on of various forms on mainframes.
Licensed Ham radio operators used telegraphic keyed Morse Code to communicate with other Ham radio operators on a hobbyiist basis. It has been more or less replaced by cellphone text messaging.
Ham radio was more popular in the 1940's through 1960's. I still have an ARRL (American Radio Relay League) 1968 handbook. It was a good reference when I was in high school, covered electrical and radio theory with both tube (valve) and transistor basics, calculations, sample circuits, build articles on making one's gear from scratch, and etc.
They even had "chat" terms like XYL - Ex-Young Lady, a reference to one's wife. They would text a sentence in code, then receive a reply in code.
WW2 and prior era, pilots military, commercial and private had to know Morse Code. Voice communication was of limited distance, code carried further. Even military fighter pilots had a telegraph key in the cockpit because CW continuous wave could cover greater distances than voice.
Hams even have contests where under very low power like 1 Watt transmitter power (sometimes called a "peanut whistle"), see how far they can telegraph someone, sometimes reaching as far as the other side of the globe.
However I would not call this "social media", as it is limited to licensed hobbyist radio operators, who are a select group. Their common interest is their radio gear.