^^^Ignore this guy lol. There's a boatload of money to be made in many different forum niches. Just like with any online venture, it takes hard work and dedication to make it fruitful though. As long as your forum niche has demand by consumers, there is money to be made.
As others mentioned, build up activity in your forum. SEO it based on specific keywords related to your niche. If it means holding conversations with yourself for a month straight just to get the ball rolling with activity, do it.