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On new facts, reliability is determined by looking at user votes weighted by their trustworthiness on the topic (votes from unreliable users get dropped). Grow from there. The system wouldn't drift much, because truth is self-consistent -- BS is not.
Facts are not democratic, but finding sources and reasoning about them can be. Wikipedia proves that crowd sourcing can produce facts.
Please hire me to do this. (Seriously! I have a msc in philosophy/history of science, phd in comp bio, and currently working in NLP 
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