Forum Marketing is an Old-School traffic source, It really works even now, in the era of Social Media Sites.
But I would like to share a complaint about this, forums allow people to get in touch with each other members, as well as using it for making money, sell services, etc... but nowadays there are SO MANY SPAMMERS around that is quite hard to use the service correctly.
Why am I saying this? I was a forum moderator for a while, and I experienced it by myself. There is a lot of people that wants to make money online, and most of them go straight to the fast and easy way... SPAM...
but what they don't know is that it does not work!
Eventually they end quitting their attempt to succeed online (maybe for the best), but the problem is that they leave a lot of junk around. Sum all of this with all the most known forums around, plus the thousands of newcomers that simply don't know anything about how to do things right... and we end up with a service that we won't be able to use it anymore in the near future, and that was a fantastic source of income for Online Entrepreneurs...
Just my thoughts, I still love forums but I think the main purpose now is to get in touch with other people, learn from each other, rather than making a huge effort to drive some traffic into our websites.