Members that I've talked to for years have either vaporized or post very infrequently.
That's the way most online forums are. Rarely do you see users who maintain a high level of involvement year after year after year; either they tire of the forum or of whatever hobby that forum is supporting. This is hardly the fault of signature campaigns.
I would say my opinion is biased as well, since I participate in a sig campaign, but it's no more biased that those who don't participate and don't like it. I actually see more spammy posts/posts that make no sense/bad google translation posts from people without ad signatures than those with.
Support the little guy, both advertiser and avertisee, keep paid signature campaigns around.
When a forum is properly curated and moderated, you see people sticking around for a long time. When you have a free-for-all like you have here, the trolls and other anti-social people are allowed to run rampant and it chases away the intelligent people that are here for proper discourse. It's not a surprise and happens all the time with unmoderated or lightly moderated forums. Without moderation, a forum will decend to the lowest common denominator, which is that of the trolls and anti-social element and that's exactly what we've seen here.
The problem is keeping a good moderator staff that has the ability to fairly and more importantly CONSITENTLY moderate a forum. Right now, this forum lacks any sort of real moderation and the moderation staff are inconsistent in their applications of what few rules there actually are. So if you want a forum that's useful and maintains users for a length of time, you need to a) start moderating heavily and properly and b) get a good moderation staff. Both of those things are incredibly difficult to do, unfortunately and even money can't buy a good, consistent staff. That takes time to build.
I have to wonder if the sheer volume of posts this forum receives has something to do with the level of moderation. There's like 20+ pages of new posts every 30 minutes, it must be difficult to review every thread, not to mention every individual post.