Adding other factors into this system might help.

What about:

    weighting a vote by the "degree of separation" of the voter? (ie., friends count more)

     hyperbolic discounting of karma acquisition 

     limiting the number of voters per item

    new accounts can't vote until achived a network & karam status (eg., shares, by friends, are upvoted). 

    accounts which primarily upvote/downvote outside of their network are ignored

    etc.

I think this adds up to saying, "small networks of voters" are the primary type of community; and big-network effects are largely discounted. Where things are popular, they are because many different active communities appreciate them.