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Human nature. It could be manipulated in a thousand different ways.

At the moment there are only 2 moderators or judges who are deciding whether your posts are good quality or purely shit.

Time and time again we see friends upvoting friends, why would this case be any different?

The problem with all these upvote bots is they are actually preying on the minnows to become richer themselves and I have not seen one yet that is true to the community spirit. This in my opinion is one of the major hurdles stopping steem becoming a real contender for top spot on the social media charts.