Skype, what's that? XP
I'm kidding, I know what it is. ^^ But I notice most have abandoned it, save for its legacy userbase who've had it for ages and still have use for it (mainly, video-conferencing and the like).
Kidding aside, it seems Discord has become the standard for everyone, regardless of the user's age. At least, that seems to be the case for most from Western EU and especially North America. I only cite those regions specifically because I'm from the U.S. & have used Discord since Summer of 2016...just shy of a year since its launch, and most of my RP partners from elsewhere and the couple I've gotten here so far, are from either / or part of the world....all whom use Discord. Most others I've read use some app called 'Telegram' but that's rare, and seems to be getting more so by the day.
There are/were other apps, but they've either shut down (Kik, several others), or are mostly used by under 18'ers. (Snap, Instagram, etc etc....most are geared towards 'drag/drop' image & video posts; not suited for text/IM.....too clunky for that).