I'm not so sure. I got us using Slack at work, and that was after about 12 months of hearing ads for it. It is IRC plus a slick UI and integrated bots, but putting all that together with a web UI and a dedicated client and phone apps makes it multiple times better than regular IRC for the common person.
I don't use twitter. I only started actually using my Facebook account I've had for many years last week, after finally deciding it's curring me off from people to not use it. I've never used Instagram or Snapchat or any of those other newer things. I'm not what you would call "people willing to switch to the next big thing". Quite the opposite in fact. At this point, the new users of slack are hearing about it from other people that are themselves 3-4 people removed from those early adopters.
That said, Slack isn't a perfect snowflake. It's a simple idea executed well. The reason I was so for it once I finally bothered to check it out is because my prior experience with IRC let me immediately see the value they had created by combining IRC and all the extras for IRC together in a nice package, and made it accessible to less technical people.
> The reason I use Slack is because it is better than any of the alternatives, but I wouldn't hesitate to switch to something better if it comes around.
Same here on why I use it, but I would hesitate switching (given most my usage is work related). If Google had a good competitor, we would probably switch, since it was our solution to crappy support for what we need in Hangouts, as all our other stuff is in Google Apps. I imagine a lot of people/companies might feel the same way with MS if their stack is dominated by MS products. For those that already have a fairly heterogeneous environment, there's probably less reason to switch to the major names, but I imagine those groups are already fairly on board with trying new tools to find what works best, so have little allegiance.
Edit: s/seeing ads/hearing ads/. I think a really good case could be made that Slack was genius to market on podcasts as much as they did, and on the ones they did, and that might have been instrumental to their growth.