It appears awareness is your biggest problem. Do you have Social media accounts that represent this website? If you do, I would recommend posting daily updates on Instagram, Facebook, Twitter.
Instagram: This will be more visual focused, you can either create infographics with information related to the many topics on your forums any picture that can relay some of the information from your site in an easy to understand fashion.
Facebook: I would recommend creating a page for the forums, I would follow the 80/20 rule meaning 80% of the content you post is other sources from Wired, huffington post anything that relates to your brand. 20% of the time you post your content, while 20% may not appear like a lot it will save you the risk of looking too much like spam.
Twitter: Similar to Facebook, i would recommend posting with the 80/20 rule and articles accompanied by images. On twitter, people are far more likely to notice a tweet if it has an image associated with it.
Blog: If you feel like you can maintain it, a blog would be a great way to gain more attention to your website, besides the fact that it will allow for google to find you better, it will add more incentive for users to come to your website.