This is just a wild idea without much thought, but I used it on an old web site I maintained that had social interactions.  Perhaps you could make a similar system work by giving a salary of points to members.  The longer they remain a member, the more points they get, but at the same time, you could make it so a portion of their points are transferred when they vote.  This might encourage point hoarding, though, which was a problem that existed with my prior system.  (In actuality, my system gave points for participation (visiting the site with some frequency) and removed points through a 'Karma tax' for not visiting the site, and then people could transfer points to each other.)