Slack communities is a trend that we're following very closely over at Discourse, and I've spoken about it before. After seeing countless more use cases this past year I'm probably more open to it now than I was initially, but I still think it's the wrong tool for the job in many cases. The allure of is as strong as ever.
But it’s less good for building up a useful knowledge base. The same questions might be repeated endlessly. Knowledge is lost once it rolls off the edge of the top of the page. You lose your search traffic (which accounts for 70%+ traffic in many communities). It’s harder to shine attention on your stars or build content on top of it.
This is huge, and unfortunately it's not something you'll feel the effects of until you're several months or even years into your project. I'm glad Slack is introducing the concept of a community presence to companies that might otherwise not have one at all, but I sincerely encourage every Slack community to have a streamlined process for "Post Slack snapshot to PlatformX", where PlatformX can be your forum, your docs platform, your blog; anything that's publicly available on the world wide web.
Slack is an amazing tool for engagement, but in our excitement we must not forget why we built this WWW thing in the first place:
The World Wide Web (abbreviated WWW or the Web) is an information space where documents and other web resources are identified by Uniform Resource Locators (URLs), interlinked by hypertext links, and can be accessed via the Internet.