I can't speak to ad revenue, but I helped with managing some large ~100k member forums and the cost wasn't by any means prohibitive. Costs today would be cheaper by a couple orders of magnitude. Spam wasn't a big problem either. Basic filters, and mandatory intro posts and post count restricted forums solved most issues. Moderation was much more focused on keeping things in topic and redirecting duplicate topics.
Yeah you can easily host a phpbb or vbulletin or whatever still exists these days on a single dedicated server (ovh or hetzner) and be able to serve 100k MAU for about 50-75 USD/month.
Hell, for less than 10k MAU you can still get away with shared hosting if you're really strapped for cash, 5-7 bucks per month.
People around here really lost perspective of what can be accomplished with a dedicated box and a PHP forum.
You don't need kubernetes to handle 12 AWS instances in 3 regions, with S3 storage and distributed RDS with a side dish of lambdas to run your 45 microservices.
Few people seem to need custom-run forums when a plethora of subreddits exists! It's the network effect: everyone interested will head there, because they know that everyone else interested will head there.
It's relatively harder to make your forum even known among potential interested users on the background of the entire Web, while you have a much narrower search space within reddit.com. (Widely popular Facebook groups also exist.)
The quality of information on reddit or facebook is so inferior though. It's really disappointing. I think it's a combination of youtube, reddit, facebook and instagram. That interests are spread out and each of those is individually vulnerable to spam for purposes that have nothing to do with the actual topic.
I'm in a carseat facebook group, and people constantly post engagement spam, I'm really not sure to what end, but I assume it somehow makes other facebook posts visible. Then there's the run of the mill scams that hit all kinds of networks.
"My Autistic Son loves 'Yamaha Boat Outboard Motors' and made these T-Shirt. Please take a look and let him know what you think, he loves comments~~~"
Then I get tons of youtube videos for topics that have all the keywords I need but are literally not even working on the same vehicle or problem I'm looking for, but of course you can't figure that out until 3 or 4 minutes into the video.
The few dedicated sites I'm still in that are active are far and away better than their reddit /facebook/ instagram /discord contemporaries. Though have still declined from the 2000s.
At least you can search reddit with google. Facebook you can't find anything, sometimes I can't even find the groups that I'm a member of.
