Slack is not a good example for this headline. Loads of chat applications are much better than Slack. Slack sucks. Slack is chosen by business people, who heard about it from their business friends and ignore input from their tech team.

Examples:

Discord - but terrible data policy as well, but at least works fluently without long load times and they managed to use webrtc in 2020, which Slack still sucjs at and gives you BS about using Firefox and limits It for the free account. Ridiculous.

Zulip Chat - Much better chat functionality and conversation model. Can be self hosted.

Anything with a proper Markdown parser, not something that sucks as much as whatever Slack is using. Oh well, but it could be worse looks at Atlassian Confluence.

Anything that does not heat up 4 Cores at 100% at startup for an inapproprIate amount of time.

Anything respecting your privacy.

Keybase (now zoom)? Matrix(getting better all the time)?

Slack was a godsend for medium to large sized hobby communities and student organizations. At UC Berkeley, the groups I was part of switched from IRC (for the nerds) and consumer group text products to Slack. There was (and still might be?) a large hobby community using it, although I think Discord largely owns the space now in the consumer market.

I use Discord for business communication.

I have a private, premium slack with ~20 of my closest friends from college, which I set up in March when we all were sent home from campus. It replaces many many disparate group chats and fits our need for customizable text-and-image chatrooms, voice and video calls, and support for mac, windows, linux, android, and iOS, with machines of vastly different levels of power and quality (although it could be a little more lightweight to help out low-tier computers).

Why pay for Slack when you could setup a Discord server for free?

A bit of an edge case, but I recently managed to move all my important conversations with friends to Discord - except for one friend who didn't use it. Since he also works in tech, we just made a Slack workspace for the two of us. I actually find it quite nice to have different channels for each topic that we're mutually interested in.

Slack and Discord are my two favorite chat platforms right now so I'm very happy with the way this all worked out.

I absolutely refuse to use Discord ever again after they started requiring phone verification. I'm tired of every company under the sun wanting all kinds of information from me that have nothing to do with what I need their service for. It's frustrating that it's become so popular.

Phone verification is the cheap and easy way to stop spam/malicious actors, plus you get a unique identified to track people with. I imagine it's here to stay in the short term.

I'm on a couple of social Slacks. The most active is me + three others, all of whom live pretty close to each other, and we've been using it to stay in touch while not seeing each other in person and also while not blowing up phones with notifications (i.e., other IM mechanisms will ping us in real time).

I think we mostly ended up on Slack because we already use it in other contexts - if we didn't, we might have gone for some other IM system.