Xin Luna Dong at Google works on the problem of web site factual credibility. See her talk at Stanford.[1] There's a database of well-known facts, and when web sites mention those subjects, their credibility is rated by detecting disagreements with those facts. This in turn affects their search ranking. This helps detect satire sites, fantasy baseball vs real baseball, Obama "birthers", and such. She said the system is live for some topics, but not fully rolled out yet.

[1] http://web.stanford.edu/class/ee380/Abstracts/150429.html