In terms of structure, having been part of Slack groups ranging in size from 2 to 2,000 people, I have learned that Slack only really works at the team level of 2–25 people (give or take). It is possible to derive value from larger channels, for example as activity streams mostly for notifications, but not as conversational or collaborative spaces.

The chat overload is just beginning — Work Futures

Agreed. Global Game Jam’s Slack channels were frustrating. IRC felt better. Basecamp was not bad. Collaboration at scale across all time zones on the palnet, both synchronously and asynchronously, is a significant challenge.