We use Discourse (almost exclusively) internally as our means of communicating and working together. We have very few private categories (we prefer to operate as a flat organisation as much as possible) but we encourage people to mute categories or tags that don’t directly relate to their work, to keep focus.
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We have been using Discourse exclusively for our internal communication for 8 years. Discourse has replaced our numerous mailing lists and has also replaced almost 90% of our internal mail traffic. We come from the event sector. We have moved our organization-wide knowledebase to Discourse. Mainly external mail communication, Excel, calendar and file sharing is realized via other Open-Source apps.
I never tire of recommending Discourse to every organization for internal communication. Only instant messaging could be improved. 
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