A lot of forums are focused on chit chat and not on content that makes them grow. Chit chat keeps your current members happy but it doesn't grow number of readers.
To come out on top...
Make sure you have Niche site.
Increase quality article-like content. People really don't care if content is an article, thread, blog, group discussion, or post. If it's good it will increase traffic, membership, and participation.
The typical forum layout/design needs a major overhaul. Get yourself a Responsive design NOW. Think mobile (smartphone, tablets, etc) and touch (desktop, mobile, etc) devices.
The main page should provide the latest/trending/etc topics. (think mobile; it's also good for desktop)
Use FB page (not your personal page) to expose your brand to the masses on a daily basis. Every post or every other post should contain a link back to your site. Use FB to inform your followers of hot/trending topics.
Most importantly do not be afraid to make changes to your site. This does not mean adding a shout box or anything stupid like that. It's about improving user experience and increasing content (either in-house or user generated).