Use Hide hack to make people reply to the posts will be a good solution ??

Probably not since they can't really see the content, and neither will be the search engine spiders..

I think the hide hack is more for hiding answers to puzzles or movie spoilers.

Well, you have a point, but if you want the forum more active, then how do you make people post ??

Read through this forum, I've posted a good bit of tips.