What are the ways to build a community? How is it different than building a knowledge base?

What does Stack Overflow want to be when it grows up?
a blog by Jeff Atwood on programming and human factors
Knowledge Management for Developers | Stack Overflow for Teams
Stack Overflow for Teams is a space for teammates to ask questions and find answers. Sharing company information has never been easier.
Are there any clones/alternatives for running a Stack Exchange style Q&A site?
Since the source code for this site isn’t available for purchase or external use, I’m looking for software that can run a Q&A environment similar to Stack Exchange. Are there any such pieces of

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