Structuring People vs. Discussions

Do categories serve a purpose? 

When you're with your friends, do you categorise discussions neatly into different areas of the room? Or time-slots within the hour?

In most communities, the 'general/off-topic' categories are the most popular.

Most people don't want to talk within categories. 

If members are looking for information on a topic, a good search box trumps a category every time. If members are going to ask a question, asking it in a category ensures far fewer people will see it. If you want to structure useful information for members to find, wikis, ebooks, and simplifying highlighting good discussions are better options.

Displaying bad discussions alongside your most valuable ones makes categories redundant. 

Categories work well in broad communities where members are likely to only participate within one group. FitBit has distinct categories for distinct goals, for example. But here they're not structuring discussions, they're structuring people. And structuring people is the secret. 

Take communities for us community professional. Is it better to structure a community by topics such as strategy, measurement, platforms, growth or better to structure a forum by people such as non-profit, B2B, internal etc…?

You are probably far more keen to interact with one specific group on many topics, than with many different groups on one topic. 

Stop creating so many categories. Structure the community by people instead.

Create people-oriented sub-groups when the majority of discussions no longer appeals to the majority of members.