my initial thoughts would be something along the lines of how an initial upvote for a post that eventually goes viral has more value than an initial vote on a post that dies... if a post has little or no votes and goes stagnant but someone's (re)discovery leads to virality of the post, that (re)discovery vote should have extra value. I'm sure there are many other ways to incentivize the behavior. That part was a bit of a last minute brainstorm I haven't thought much on yet.