The first forum I used to frequent regularly that went quiescent was the militaria section of the Swiss Rifles Forum. The sudden passing of Frank "Guisan" van Binnendijk sapped all the energy out of that board, because no one else could step in to fill the giant void he left behind. When a forum relies too heavily on the contributions of a few 'stars' like Frank, it is inherently vulnerable to this kind of shift in fortune.
Then the ICUS forum, associated with Camopedia, changed servers for logistical reasons. The transition was not very smooth; which dramatically reduced the amount of traffic, and may have contributed to the forum's apparently irreversible decline. The elephant in the room here is that someone must pay real money to keep these playgrounds open for the rest of us, because nothing is really "free". Unlike Facebook, the people who operate these esoteric forums are basically doing community service, motivated mainly by their own passion. If these guys stopped enjoying what they are doing or can no longer justify investing time and resources into their project, the forum's fate is also sealed, regardless of what the rest of us think or do.
At the risk of channeling Captain Obvious, I think most of us frequent discussion boards for the fun of it, and to receive some type of affirmation or validation that we are neither alone nor mentally ill to find pleasure in making treasures out of stuff that 99% of our fellow humans think is junk. Talking to oneself is neither fun nor validating. While the lack of response inside a mostly empty room may not prove that one is alone or mentally unwell, it certainly doesn't help.
Bottom line, a forum lives or dies based on the availability of fresh blood to sustain the "fun" factor; it needs continuous renewal to make up for the worn and tired, just like any living organism. Paradoxically, the availability and cumulative nature of a searchable archive, which makes the forum such a valuable resource, might actually work against its long term health, because over time, it will be easier and easier for new users to find the answers they are looking for in the archive, without ever making their presence known. Also, as all the low hanging fruits are already taken, it becomes more and more difficult for new participants to contribute anything of substance to the discussion, further reducing their motivation to uncloak themselves. Consequently, once the forum reaches maturity, the trend is towards more and more anonymous takers, even as the givers stop enjoying themselves and decide to head for the exit, potentially transforming the once lively forum into a perpetual zombie.
Perhaps this is why communist utopia never had a chance in hell, while Facebook thrives.