I've been using Riot.im for a few weeks, both the web and the mobile client, and...
...it's a pretty damn solid experience! Finally a well-designed, secure, free as in freedom IM that doesn't suck.
Could you all, you know, ditch Slack, IRC, WhatsApp, Telegram and Discord and move to Matrix? Onegai-shimasu 🙏
mastodon.matrix.org is one server in the network