I definitely agree with what everyone is saying so far, in that you first need to have a forum centered around a topic that'll generate traffic and discussions on its own. I guess the question though is that once you already have that established, how do you keep your users from wanting to leave to find another site - how do you keep them coming back and engaged?
for example, my site is 11 years old, has roughly 630k posts, almost 6,500 users and at one point, was receiving about 300,000-400,000 pageviews/month and nearing about 200 posts/day. I haven't checked what my pageviews are like these days, but I know my posts per day has been cut in half to around 100 posts/day..sometimes a little more, sometimes a little less. It is also summer here in America, so more people are away on vacations and spending more time out of the house... We also have more competition in my niche space now than we did a few years ago...although some of the larger sites have closed up shop over the past 2-3 years..
how have you removed friction on your site to encourage easy engagement with users, converted guests into active users and made good ui/ux decisions so that the site flows well? is your site gamified for achievements (your profile is fully filled out, you have 100 posts, etc., etc.)? do you send out a weekly/monthly digest? when someone registers on your site, do you send them a welcome email?
my current site on mybb is not gamified at all, nor do I send out a monthly digest to look back on what it going on. I plan on changing this once on flarum as there's extensions that are modern to help with this. which ones do you like and use to achieve this, if you currently do this on your site.
just brainstorming some ideas here...