I administer a geographically-specific poly forum, and I feel your pain. We had a large core group when we started, but a lot of them have moved on - usually because they have discussed all they want to discuss and are getting on with life.
We get a few that show up, post a few times, then disappear, but it's not really enough to keep the forum feeling vibrant at all.
Not sure quite what the solution is, to be honest.
"Listen, or your tongue will make you deaf." - Native American Proverb