Hi @Sam_Leahey and @Bastien_Siebman 
At Asana, we use Slack for quick or informal communication (ie: how was your weekend? / Are you available to meet on Tuesday? / I may be running late to our meeting). Most of us use the Slack <> Asana integration, so if anything actionable comes up during a Slack conversation, we can quickly convert it into a task/comment to follow up directly in Asana.
And of course, we use Asana conversations + comments on tasks for anything related to project work to make sure everyone is looped in the conversation can catch up in their own time (which becomes even more important when working with an international team like ours 
).
Hope this helps!