I tend to disagree with the social media thing.
I log on to Facebook in 10 minute spats and let's face it Twitter has become one big market place with people selling their wares.So I think a forum still has relevancy.
I would look at the "lurker" angle and encourage them to join in.I mean,who logs on to a forum and has no urge to join in and voice their opinion?
Perhaps put a temporary ban (a day/a week) on lurkers that don't post?