I say "browser-based" because Slack, HipChat, et al are chat platforms you need to use a browser — or something that embeds a browser, like their official apps

I say "browser-based" because Slack, HipChat, et al are chat platforms you need to use a browser — or something that embeds a browser, like their official apps — to access. They may have gateways to standard protocols like IRC, but those are gateways, need to be enabled on a per-project basis, and may even be extra cost features. If you don't see why I'm comparing something like Slack to AOL chat rooms and Yahoo Chat, perhaps you don't understand what Slack is as well as you think you do. The fact that you can set up an instance for a team or organization isn't what's important in the comparison, it's the scalability of the service and the degree of client resources used.