In a previous life, I used to be BIG on forums. I'd buy vBulletin license like there was no tomorrow.
My first car was bought from monies I made from selling a forum.
THE NOSTALGIA! Ugh... F you Google!
Hiring forum posters to post arbitrarily used to work well before. Now if you're building a forum, you have to treat it like any other website. There's research involved.
1. Scrape PAA - Figure out what questions people are asking. Get your posters to initiate threads about this.
2. Gain Topical Authority - Create Guides and Resources about your topics. Let people respond and make a healthy discussion.
3. Build Backlinks - Build backlinks to your resources.
4. Show related posts - These function as contextual internal links
5. Steal Users from Other UGC Portals - People who are part of communities - FB Groups, Pinterest, Reddit, Quora - Participate, then mention something like.. "A very interesting post I found was at forum.com/showthread.php?p=4545"
This is where you should start seeing traffic.
Once you have that, you can figure out sign-up magnets and urge people to sign up.
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There was a time when hiring forum posters with "50 words per thread and 10 words per reply" used to work.
Now you need to create very informative first posts, or if the first post is a question, the response should be very informative.
Good luck bud.