There’s something tragically ironic about community builders feeling lonely. 
People come for the skills and stay for the feeling that they made the best decision of their lives.
This is almost word-for-word how I explain my experience with the Ruby community! “I came for the language, and stayed for the people.” 
On the ROI side NPS sounds like a good idea. You can have any amount of stats about the community itself, but you can’t show it correlates with business outcomes it might all be moot.