Back when I had my original site, before I got into casinos… I grew a forum from nothing to 1000s of posts per day with 10,000+ members. It received more that 90% of posts on the web that related to the website topic, taking most of the traffic away from all of the existing forums.
Some of the key elements for growing the forum were likely:
1. Having the top Google ranking under the three most related Google search terms, and getting most of the new traffic
2. Having a superior message board format, including both message board type and forum selections
3. Timing… the largest forum from before the days of my website went under. The posters from that forum knew about my forum.
4. Having a few very high quality posters that were not on other forums including industry reps and retired former workers in the industry (the latter had much more interesting and informative posts)
5. Having more interesting discussions as a whole, many of which were started by me in the early days
6. Having more relaxed forum rules than elsewhere
Having said that the forum was an incredible about of work, and not particularly rewarding. I have not added a forum to my casino sites because of this.