Humm... good question.
I'll offer a couple of thoughts.
1) It's summer. DRN is slower in the summer months, while a lot of other boards seem to be more active in the summer. Go figure? I suppose that since the forums are not focused on discussing things like the sport, ie; MX/SX gossip, etc., that would make sense? I could be wrong. DRN'ers tend to ride rather than post in season.
2) We average (over 12 months) 800 or so posts per day. Too much reading for me as it is.
3) We limit the number of posts non-subscribers can make each day... it's really cut down on the useless drivel (and post count) that used to clog the board. That's not to say that non-subscribers don't have good things to say or good questions to ask, that's just saying that when folks know they can only post twice a day, they tend to keep the two word sentence or basic BS replies to a minimum.
4) We aren't about how many posts we can log each day, we are about good quality members, making good quality contributions. I'll put DRN against ANY forum on the web for quality content and "community". The "good deeds", the charities we support, the helping hand given to other members is unmatched. End of story.
5) The dreaded Chat Room set up for the DRN STM's also cut the post count much more than I would have thought... several thousand lines of chat each day, lol.
Yes, the number of members since 1999 surprises me, but really makes no difference, nor do the number of posts, the quality of the community is what counts and what makes DRN what it is. You can't make it, you can't buy it... sometimes it just happens. In this case it did.
50% of the membership has or does post. 7,800 members have posted in the last 4 months. The other 50%? I suppose they read, learn and enjoy. Some join to ask one question. Some come and then go, who knows why. Some dislike the way this board is run and go elsewhere... it's all good. 160,000 distinct users visit the site each month and there are still only 30,000 members. Who knows? Know someone that runs a forum like this and has all the answers? I'd like to discuss it with them.