Not sure, but I really don't believe Slack's numbers either. I have a Slack account that only occasionally gets used, so they probably count me. Whatever the real numbers might be, Teams is very heavily used. My university has Office 365 and Teams comes with it for free. No need to pay for Slack. No need to expect others to use a new tool and a new app and a new login when Teams is right there.

> This allows them to maintain a "familiar" setup to everything else they have.

What's important is that when you log into your email account, you're logging into Teams, Planner, etc. I recently started using Planner, as awful as it is (seriously, you can't even drag cards!) because I can't expect others to adopt a new tool.