I'm somewhat surprised that Amazon didn't end up acquiring Slack, considering that the major thing they are missing compared to Microsoft is productivity tools. If they had purchased Dropbox and Slack they could've gotten a foothold rather quickly and slowed down one of Microsoft's major selling points (you already use Office, why not Azure?). Discord could be another interesting company in the chat space, as I'm sure they are considering an enterprise play (if they aren't they are insane, considering they have one of the more user friendly chat services and with SSO and a SLA they could probably charge $10 per user).
(Yes, I know Chime exists. But I don't think I've ever heard of anyone using it, and Slack is still rather popular.)
(Yes, I know Chime exists. But I don't think I've ever heard of anyone using it, and Slack is still rather popular.)