I always keep checking the dark eldar part of this forum as it's my main army, but the last few months/years it has been pretty dead. Pretty sad, it's always awesome when a codex comes out, or a new edition, always see more activity.
Another big factor is the size and the type of army you're building. A guy who builds a 7k force is mostlikely a guy who buys at least 1 units of each in his codex. Such a force gives a lot more possiblities to test. I'm one of those guys, you go to the GW, you ignore FOC, just take whatever units you want to buy, even if they're bad ,even if you already filled that particular FOC slot. So many don't have the budget and they build 1 specific armylist, I know from experience that gets old fast and they quit ,put that army to the side, until a new edition, or a new codex.
How big your army is and how well you paint also matters, if the army is big and the guy wants a kickass job on everything it will take time to do it and and he'll stay busy for a good while.(I'm actually like that type, got several huge armies to keep me busy)
It's not just the dark eldar section of this forum, it's most of the forum that has been pretty slow, that's what has been keeping me away. I see so few posts in the section I like to visit, when you post a reply or post a new topic, it takes forever for a reply. The slowness of the posting, but mostly the lack of posting is what is slowly forcing me to quit to forum, I feel less and less like coming back. So few are posting it's sad.
Another small, but still important point: I'm mostly against spam, but not completely, in the case of painting I am not. If I work on a guy for a month, I post pictures: I post it, for people to see it, I would expect some replies, even if it's a nice, not having any replies is almost like people are ignoring the post. It's a bit of an ego thing I admit, but it's always nice to get a compliment and it encourages me to paint more. It's nice to have a constructive comment, but a compliment is nice as well.
I did notice that this particular forum has less activity, I see plenty of activity on other forums and on facebook pages. I look mostly for Dark Eldar related posts, and I get plenty of posts, I know that IRL plenty of players either play DE or they some of our units as allies, plenty of DE activity, weirdly not reflected on this forum, but it is on most other forums I am on.
These days I'm mostly all about converting and mostly about painting, this forum has close to 0 activity on that side, while I get plenty of action on other forums and on facebook. I post a picture of one my latest here, maybe in 2 days I'll get 3 replies (usually come together) On facebook those 3 replies will come in a couple of minutes/hours. People are a lot more active on FB than on forums. The number of likes is also more important that the number of view. Whether you reply or you leave a like matters. When you look at something and you don't leave anything behind, it usually means you either don't care or you didn't really like what you saw.
I don't need to make a dramatic goodbye like others have, most people have left this forum anyway, but I honestly don't feel much like hanging around on this dead forum anymore, I mostly waste my time on here looking for good threads, replies take forever, while on other forums/BF pages, there's plenty of activity.