What do you think about the idea of having a forum post of the week (or month)? You could go over the new discussions created during the week and nominate a few and post on a sticky thread for forum members to vote on. The winning thread could be showcased for that week as a sticky thread. Or, you could simply pick 'the best of' type discussion for the week in every subforum. Though there would be no prize for winning the forum post of the week, it would provide a moment of pride and recognition for the person who started the discussion which might encourage them to compete again.
Deciding on the best post of the week or month may be hard, but the idea to award good on-going discussions is worth trying, in my opinion. In a forum I'm a part of, we make featured sticky threads once in a while to give exposure to the content provided inside. The acknowledgement can be used to encourage members to brainstorm even more topics to chat on and enrich the forum's content.
I think it can, in a small community, create a bit of envy and accusations of bias. After all, forums are all about subjectivity & opinion. Among blog posts, poetry & stories it's a bit easier to select winners, as this sort of content has more objective value
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