I'm reviving r/ruralplanning and I made and shared a list there of various subreddits related to any kind of development or sustainability work. I am sharing the link here for those that are focused not just on finding a job with a humanitarian dev agency, but building their awareness about issues related to the work of international development and humanitarian response.

In addition, on Reddit4Good, I have a list of various subreddits related to humanitarian work: public health, environmental work, refugees, etc. (it's near the bottom of the list).

Suggestions for additions are welcomed: what subreddits do YOU read related to your area of expertise?