From a recent IM chat:
JATB: you click on “bite some chumps” and it shows all your friends who aren’t zombies yet. So you bite ’em.
JATB: If they add the thingie, they become zombies too.
JATB: And you get points for it. Then you also get points when THEY bite people. So it’s kind of like Amway.
Actually, now that I think about it, it’s exactly like Amway.