When I started my forum in 2005 it grew at an alarming rate -- before long the server started crashing and we had to move to VPS hosting.
At the height of its popularity it was getting over 3 million unique hits per month, with so many people logging in we needed a good ten moderators to take care of them. If anyone bailed, caused trouble or started a break away forum, well it hardly affected us at all.
Move forward to now -- fewer than 60 people log in every day, I moderate it alone, struggle to make enough money to pay the hosting costs and (at the time of posting this) the last post was two hours ago.
I have to admit that moving to Woltlab seems to have had a negative effect on traffic for some reason -- in spite of its cool look, quite a few people seemed to dislike the interface and have jumped ship as a result. It's being picked up by search bots, so SEO doesn't seem to be an issue.
While I know there are still niche forums that people enjoy, they all largely seem to have taken a hit with the rise of Facebook Groups etc.
The question is, how long can forums be sustained before we need a complete rethink of how we run independent discussion boards?