No matter what type of forum you run and what niche you are in, there are many ways to earn money with it. Think of how many forums start out even without the idea of making money and can have so many advertisers begging them to put their ads on their site. Or think of how many affiliate related or CPA related offers you could put on the forum? A forum is kind of like an email list, except you have stronger relationships and a lot more interaction, so the conversions can be huge.
Learn how you can set up your own profitable niche forum at: Niche Forum Goldmine.