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I think lurkers will always be lurkers, a notice won't make them post.

I would recommend giving something for people with x number of posts or trophy points, e.g. resources to download or other content that is not just member only, but only for members with x posts, or y frequency of posts (not sure if that's possible, but there must be an addon)

Above all keep the forum active, there's nothing like good content to get people joining in and sometimes the forum owner must be pro-active in getting the discussion started and flowing.