People liking IRC is a reasonable conclusion to draw.
Mine happens to be Facebook Messenger.
You are mistaken: FBM supports multi-way communication. For work, I use Messenger almost exclusively. For personal stuff, some combination of Messenger, Google Chat, and SMS/MMS. I do log on to IRC once in a while, mainly Freenode, but it's far from "primary".
Why does the "average" person ever need weak identity communication? I mean, I'm glad it exists and it has a niche, but why would you expect that to be someone's main way of interacting with people?
Anyways, there is an issue with FBM's multi-way: It's designed as an invite only system: you have a list of people you want to talk to multi-way, and you talk to them. This is fine in certain contexts, but in other contexts, the "room" abstraction used by IRC is more effective, as it allows you to have conversations with a strictly defined topic, but not strictly defined participants, the opposite of FBM. FBM's system is fine for small-scale multi-way communication between known parties, but that's not what people use IRC for.
I doubt it. I see less and less developers using IRC outside of quick help and instead using Slack.
A lot of languages and techs have both a Slack and IRC... it's hard to know which one's more active sometimes which is annoying.
Elixir (language), Ember (JavaScript framework), Elm (language) all have more active Slacks than IRCs. Facebook's React moved from Slack to Discord, and the community has never cared for or used their IRC channel.
There's still fairly active IRCs like the JavaScript, Go, Clojure (I think), and the Node ones, but they're becoming the exception.
I think IRC is popular in your group of developers and interests, but for many people it's somehow considered ancient. On an interview I once mentioned IRC and the interviewer (in his 40s) was surprised it was still a thing.
Then again, look at the stacks you're looking at: they're all relatively new, and most of them are frontend, and thus more likely to jump on the new shiny. ##c on freenode has >1k users as I write this. Although that's probably also an outlier.
Personally, I'm hoping that Matrix becomes the new standard, if we ever get one beyond IRC. IRC is showing its age, and Matrix seems to be the most practical and levelheaded group doing work on the problem.
As for Slack, I'm pretty lukewarm on it, and kinda prefer IRC.
Meanwhile, Elm's Slack channel has more regulars than #node.js.
IRC is kept alive by a small holdout. I'm one of those people, but it's no mystery to me why nobody else uses it when I'm paying irccloud.com or running weechat-ncurses on a VPS just for basic features everyone expects from a communication tool.
[1] I'm curious as to whether that IRC channel is the only "blessed" Rust chat out there. The problem with other languages is that the community makes a Slack for them even though there's an IRC channel for them already and most new users start only going to the Slack.
I've also heard of quite a few very active JavaScript Freenode channels, though Slack does have a lot of inroads there depending on which particular segment of the community you're into.
And each person needs to care enough about IRC (e.g. run a VPS) to receive pings or messages while they are away and to see the message history when they return.
Maybe IRC is "plenty popular" which apparently means popular enough to find someone willing to shoot the shit with you if you're in one of its largest channels. But that's a far cry from "IRC is everywhere".
I use IRC every day. I'm just not left scratching my head when I find out that yet another ecosystem chose Slack/Discord over IRC. It blew me away when I saw how active Elm's Slack was while #elm is dead. But that's pretty much how it is with every community I'm part of with a Slack channel.
Sometimes people here suggest "maybe IRC is just for developers", but you can't even format a code block on IRC. :)