People never work for free. They say they do, but it’s a sneaky lie.
They’re really working for a reason. It might be obligation, recognition (“hey, I’m working for free!”), prestige or power. Did you volunteer to help Obama or did you volunteer to tell your friends you’re working for Obama?
If you want volunteers to help your community, you need to make the volunteer spots sacred and powerful. Why not have 6-monthly elections for volunteer spots. Limit the number of openings. Make it tough to become a volunteer. Have strict rules (you have to have 300 posts and 500 thumbs up).
Or don’t do any of this. Keep the names of volunteers a closely held secret. Headhunt and recruit key people in the community, quietly. Who moderates the forum? A secret helper! Only the insiders know who the insiders are, and they wont tell.
Or rotate the volunteers. Everyone does it for a month and then rotates. It’s like jury duty. Some members are drawn randomly from a ‘pool’ of people. You have an obligation to try hard. Do a good job, your peers are watching.
There are thousands more ideas than these. Be innovative and try several approaches. Find what works for you.