Discord is eating the (chat) world, in the same way that Facebook has eaten social blogging, and Google has eaten email.
I'm running a small site where people can play pen&paper-style roleplaying games together in small groups, featuring persistent channels and a wiki for each channel, in addition to the core dice rolling and chat functions (https://rolz.org for anyone who's curious). The site has survived the stunningly well-funded and abundantly feature-rich Roll20.net, because it's non-commercial and doesn't require user accounts. However, since Discord appeared on the scene, traffic has been declining massively and steadily.
Where we once had hundreds of groups online at a given time, it's now usually in the low 20s. At first, people kept asking for a Discord-bot version, and at least one user made one using the public API, and now Discord is pretty much the only game in town.
Meanwhile IRC is also pretty much dead, as many people have noticed in this thread. I still hang out in the #ludumdare IRC channel, but over the last two years that community has dwindled from hundreds of users during the Compo to maybe ten, and from dozens of users during the rest of the time to maybe 2 or 3 being online per day. While the LD Discord has replaced the IRC channel, it is also a lot less vibrant and social. It's now more a loose collection of mildly disinterested people who sometimes hang out in the same chat room, mostly unconcerned with each other. I wonder how symptomatic that is.