We prioritize posts based on shadow rank first, expiration second. We also limit daily up votes per member to 2 posts, so any given Shadow Caster can only get 2 posts up voted by another member in any 24 hour period.
"How does ShadowBot decide what to vote on?
ShadowBot works on a pool mentality. Every Shadow Caster is given a position in the Shadow Pool prioritized by their Shadow Rank. Authoring high value posts will increase your Shadow Rank leading you to see more of your posts receiving priority in the Shadow Pool."
https://shadowbot.us/shadowbot/faq.php