I have always been curious of this. I know that they can monetize it by ads but what else?
I don’t think they earn from posts. That would make it too easy to make money.
[FONT=verdana]A forum is no different from other websites in this regard. It can make money by showing advertisements, by providing affiliate links, by selling a product, or similar ways.
Not all forums exist simply to make money. Some companies set up forums as a vehicle for supporting their products, or simply to promote discussion. But the above methods can still be useful, if only to pay the development and hosting costs.
As for how much a forum can make, that would depend on the number of members, the subject matter of the forum, and on how hard the site owners are prepared to work at it.
Mike
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To add to Mike’s List…
Some forum sites have public and then ‘paid for’ thread areas. The ‘paid for’ areas generally have support staff that address the questions promptly and with definite knowledge regarding the product or service represented in the thread area.
Steve
i think that the main source for forums is advertising as they do not get money for posts. it is obvious.
I have website , and I think forum can make money so well as some websites . Forum owner can sale links and articles with links , can make money by partner websites (online magazines , referrals or other way ) , or by Google ads . There are a million ways to make a money , there isn’t any different you have forum or website , you just need traffic .
Forums have an edge of instant crawl rate as updates in the form of posts are noticed by the Google bots and so they index each page of the forum quickly. This in turn increases traffic over the website. Now the earning factor is related to ads and premium membership.
I had a social forum once and i was making 30-40usd / month from google ads. Although i am sure its not the best way to do it, but i was a student back then so couldnt complain about free hamburger per day 
Its also the easiest way, cause you dont need to set anything up besides google account.
Nowadays its mostly what Mike and ServerStorm said.
A big disadvantage with trying to make money from forum ads is that the majority of traffic is repeat posters that develop “Ad blindness”.
Traffic is required, a vast amount of traffic to earn very little.
We earn all our forum related income from advertising.
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That’s a good point. By the same logic, a site that has a lot of highly informative articles, on subjects that people are likely to spend some time studying but not necessarily re-visiting, is likely to do better in this respect. That’s certainly been my experience.
Mike[/FONT]
I dont think that you can earn by forum posting , forum is a way to get backlinks.
Many years ago I administrated a fairly large forum. It was so long ago that I forget if they had 250k users or 25k users, but I certainly remember only earning about £50 a month from banner ads. Another forum I moderated with similar user numbers was earning ten times that though, so my anecdote is purely that I suppose.
All I’ll say is that it’s very rare to be making a ton of money from forums. I can imagine SitePoint making a fair bit, but there aren’t a lot of forums of this size and stature around on the Internet and SP has over a decade head-start over you.
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Pity we don’t get paid every time somebody repeats this old myth, because then we would be rich. 
All external links on SitePoint forums are nofollow, which means they’re useless if you’re simply looking for backlinks. Many other forums have adopted the same tactics, driven by the number of people trying to spam them just to obtain backlinks.
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Membership or revenue from topics that will be sticky.
Maybe it depends on the earnings they get from ads on their site. I mean, how else can they get a revenue. I am a member of one paid to post forum. I haven’t been posting regularly. But I was able to receive my first pay. Just think about this kind of forum. They pay people to post on their forum. I just can’t understand how they finance their forum as they also regularly pay people out.