Originally answered Apr 23, 2017
From the Communities Manifesto:
Communities form around people who share a common specialty or interest. Organizations share some characteristics, but they are not self-forming. Communities exist to help their members better do their jobs and to deepen their skills and expertise. Organizations exist to get work done.
Some organizations try to align communities to the organization structure. They try to control communities from the top and assign topics, leaders, and membership based on business unit, function, geography, client, market offering, or initiative.
Communities should be based on topics which use easily-recognized terminology, not on organization structure. Communities should be organized around industry-standard, universal topics with which members can identify in their specialties and roles.