I used to get on and there would be at least 5 other people on as well. Know you're lucky if more than 5 people log on each day!
I wish I knew how to do screencaps on my computer. :< At the time of writing this we have 5 members online!!! W00t! xD
Anyways, I do wholeheartedly agree. I think another reason for the steady drop in 'popularity' could be that at the time the forums were going strong, Neon had a web-comic or two going as well, which constantly brought new people to peruse over the boards, some of which stuck around to contribute.
Since those comics have been discontinued for quite a while, the community has been left pretty much on its own resources to grow. The members grew older, the bots grew sneakier, the board had to be moved which I'm sure lost a lot of the previous members. Most of the more prominent and oldest members have left NDA for a more advanced and challenging art forum as well.
I've been a member here for quite a while, though I was never extremely active, but I do remember times when there were so many people on at once, you could actually have running conversations with them without having to wait two days for a reply. I even remember a time with a Role-play section, which was quite popular (and relatively intelligent as RPs go) as I remember it.
I'd have to say the best way to grow and build NDA is to promote it. It will only get more populated if people actively seek to improve it. If people ignore it and just hope for it to grow, it will die.
Although..with more people, comes more problems. I recall a severe pornography incident just a short while before the forum was moved, along with the bots swarming from every conceivable direction. A problem that we don't have to worry about here too much due to the size and general age of the members here, plus the registration limits Neon has set to stop the bots.