Yep, it's true... I'm building a forum and I'm seeing how hard is and how much time it requires to get a top forum. In the next months I'll answer to myself if forums are profitable.
you need to build your forum active members. People mostly will join it as well if there find out that lot of post in a forum . If the forum which do have few post and few active user. They would not continue with you because they feel that no one will give them feedback as well. To get a large traffic forum, all is depend on admin hardwork. Admin do need to market their forum well and did some SEO. Not only that, admin to to participate in their own forum as well. Startup maybe hard but once you have lot of traffic, you will earn money non stop and you only need to manage your forum will do.
My experience with a forum is very bad. I started some forums but failed. It is very difficult to start an active forum with new members. If you achieve 1000 members and a handful active members, then it is very profitable.