Imagine Slack's answer to this, which is "network effects within teams and, more subtly, between teams. Slack will be proposed by your engineering team because their OSS projects use Slavk, their conferences use Slack, their last client used Slack, etc. Then after your entire corporate brain uses Slack for its synapses we, not to put too fine a point on it, own your soul."
Ok, but slack is just like campfire/hipchat/yammer/etc and none of those have come close to the outcome you're proposing. So what does "your" product, Slack, plan to do to actually achieve this, besides just being better than the rest?
"We're going to take $340M in funding so that we build every conceivable integration and API into the product, blast the hell out of every marketing channel, and ensure that there's never a reason to use anything besides Slack. And if you do, you're stupid."
More seriously - network effects exist when there are reasons to choose your product other than what you, as the company running it, have personally built. People shop on EBay not because it's a great site (it isn't), but because if you want to find someone selling a rare item, you are far more likely to find it there than anywhere else. People stay on Facebook (despite oftentimes hating them) because their friends are on Facebook; it has nothing to do with what Facebook does, and everything to do with the behavior of their friends. People use Whatsapp/Line/WeChat/Hangouts/iMessage based on what their friends are using, irrespective of the relative technical merits. People stay at AirBnB not because it's a great site (though IMHO, it is), but because they're far more likely to find a nice place at an out-of-the-way location than any other hotel search.
The office messaging market - for whatever reason - has started to exhibit these dynamics. People setup Slack channels not because it's a good service, but because their employees encourage them to setup Slack channels. They write plugins for Slack not because they want to, but because that's where the audience is. They stay on Slack because that's where all the plugins are. Maybe these were intentional moves that Stewart Butterfield carefully planned out, or maybe they were just accidents of history. From an investor's POV, they don't care: the fact is that people do ask their employers to setup Slack channels, and they do get on Slack because other groups they're part of are using it, and that dynamic will lead to a monopoly that the investor wants to be a part of.
How is "just being better than the rest" not a good enough answer? Better marketing, better features, better marketing, better marketing(!!!). Better than the rest, but what more do you need to be better at than convincing more people to use it? That's what makes a monopoly.
Well, that's kind of the point. Slack has built a moat. IRC is libre and free, but also can get far more arcane than the RFC suggests and to get something like Slack's feature set, you have to either build or strap together a lot of moving parts. Discourse is targeted at gamers and is not good for businesses. Ryver is way behind. MS' offering is laughable. Slack also has exactly the kind of differentiating features that don't particularly matter to ICs, but which corporate values highly—like auditing, retention policy, and permissions stuff. There's a specific subset of "good product" which has the word-of-mouth marketing result that Slack gets, and that's not the same as being "good" in a vacuum.
People don't use the alternatives to Slack because they're not as good as Slack, on various axes, is the short version. Slack has made themself hard to compete with.
I use Mattermost and don't miss slack except for mobile apps. They released an OSS iPhone app for it recently and the Android one is about to be released.
Self-hosting, unlimited history, unlimited team size. They have great deployment docs for various methods and it was pretty easy for me to containerize it for docker.
It has a slack compatible webhook integration so many slack integrations will "just work" with mattermost, if the integration will allow you to specify your own url.
