If you use Slack, I think the admin panel already contains the number of messages in channels VS DMs. Long time I last saw it myself, but I think it was missing a breakdown on how many members of the Slack received the channel/"public" messages (as not everyone is part of every channel, 2 member channels vs 200 member channels for example), maybe it looks different now.

> 5. Slack Is Great But (SIGB) - teach people that they don’t need to read everything. Many people get overwhelmed

I think this happens when Slack is the "source of truth", because the ephemeral feeling it gives since it's a chat ultimately. If you instead use a wiki/whatever to actually collect things that are important, there is less stress about possibly missing out on important things. Make summaries by week/month and it'll be even easier for people to catch up easily, which means even less stress :)