Written in 1960, Cordwainer Smith's charming (and only) SF novel, Norstrilia, might contain the first usage of "Instant Messaging". Wouldn't surprise me. There's so much in the book, amidst the cat people, thousand-ton sheep and assassin kookaburra, that is out of step with the times -stepping forward from the times- that the author pulls off a very convincing performance as prestidigitator.
He coins instant messages as "instantaneous, interplanetary communications," which is sort of similar to what we have on the 'net today. Not yet between planets, but instant nonetheless. They are mentioned twice, hardly a major plot point.
Prognostications aside, I do recommend the book to fans of the serious-minded SF, for elements of future politics and economies. While not aspiring to space opera, like Asimov or Herbert, elements of galactic adventure are plentiful. Mostly it's a tale of a boy who buys the planet Earth and his encounters with colorful characters who would be at home in a romp, Douglas Adams-style.