I run a forum in a financial niche with about 5000 total members where roughly 6% pay a monthly fee for extra features as well as access to a specific subcategory. Monthly revenues are around $5000. I started the forum 7 years ago in a very small niche that made it the best place to find content about that niche. I’ve since widened the scope of the site, but having that initial, highly active userbase was also crucial to build a revenue generating forum. Subscriptions started at $5/month but are now $20/month. So yes, it is possible 
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