Just to further that... we had a strong user based BEFORE we had a forum. Part of the reason we opened a forum was because we needed a support venue for our existing users that were already using our tools.Then you have people like DigitalPoint, who are expert coders themselves and can take a software platform and write their own custom add-ons that people really want to use, so they join and thus a forum can make enormous profits, depending on the niche. If DigitalPoint forums, as an example, didn't have such coding expertise behind it, constantly evolving the site with features you can only get there, then it would have died with the majority of other SEO / online marketing type forums that have died to near zero growth, as that boom has now gone.
The actual name for it is called, "Link Bait", which means someone creates something so unique, it goes viral and is used heavily, its something unique that isn't everywhere else. If you view DigitalPoint forums, as an example, it has its own advertising network, directory, marketplace and webmaster tools, which back the forums niche market and keep users coming back / sharing links from the site with others around the world.
You have to constantly evolve if you want a forum to explode with traffic. If you don't have those skills, which I certainly don't, then you have to use what you have to create uniqueness that attracts.
Most notably this one: http://tools.digitalpoint.com/tracker.php Even though it's now rebuilt and consolidated into the forum framework, it existed before the forum did (and have tens of thousands of people that used it every day).
Before we had a forum, we tried to have old-school email lists as a sort of public support venue... http://www.digitalpoint.com/lists/summary-1.html You can see it pretty much ceased being used when we opened the forum in March of 2004.
The bottom line is a forum is no different than any other site when it comes to retaining users... give users something unique that they want and they will keep coming back. Any site that sits stagnant is doomed in the long run.
digitalpoint.com is constantly evolving... A rough timeline of some interesting things (these are just the major things I can think of off the top of my head):
2003 - I made our keyword tracking tool one afternoon because *I* needed it. Showed it to some friends and they loved it. Spent the next day making a registration/user system so others could use it (it never occurred to me that OTHERS would want to use it... haha)
2004 - Opened the forum because we had so many people using the keyword tracker... was the buzz of the marketing/SEO world back then.
Two months after opening, I had an interesting idea... why don't we do a revenue sharing system? I was actually the one that came up with the concept of AdSense revenue sharing. http://forums.digitalpoint.com/showthread.php?t=519
Even Google since adopted the idea: http://code.google.com/apis/adsense/host/index.html
2005 - I was the one that invented the idea of co-op ad/link networks. This was great fun for a few years... we quite literally ruled Google search results in anything we wanted (for example, I decided to outrank westernunion.com and eharmony.com for their own company names)... it took literally 5 minutes of work. Google actually had to make quite a few algorithm changes over the years directly attributed to the ad network. Copycats pop up everywhere a few months later.
2006 - I thought the old (now defunct) Digg Spy was pretty neat... so I took their idea and applied it to the forum. http://forums.digitalpoint.com/showthread.php?t=101069
Had the idea for our realtime user map: http://forums.digitalpoint.com/usermap.php
Based on demand, the usermap spawned our geovisitors tool: http://tools.digitalpoint.com/geovisitors.php Which is now used on hundreds of thousands of websites. For example, I just happened to see it on a random site I was looking at earlier today: http://leandrovieira.com/projects/jquery/lightbox/
2008 - Marketplace (now one of the largest domain/site marketplaces out there - http://marketplace.digitalpoint.com/
2009 - Phone Home system to recover stolen computers - http://tools.digitalpoint.com/phonehome.php
2010 - Digital Point Ads - http://advertising.digitalpoint.com/
Sphinx Search for vB4 (more or less a must have if you have more then 1M posts) - http://products.digitalpoint.com/vbulletin-sphinx-search.php
I have a list of neat things I want to build for my site that is about 4-5 years worth of work for me... I have plenty of good ideas for the foreseeable future. The only limiting factor right now is how much stuff I have time to build.
The point is, if you don't keep a step ahead of everyone else... well... it's just much harder to keep users. Users are a fickle bunch, and will leave your site as soon as there are better options for them.