As a digression, the definition of knowledge as justified true belief runs into the Gettier problems:

    > Smith [...] has a justified belief that "Jones owns a Ford". Smith 
    > therefore (justifiably) concludes [...] that "Jones owns a Ford, or Brown 
    > is in Barcelona", even though Smith has no information whatsoever about 
    > the location of Brown. In fact, Jones does not own a Ford, but by sheer 
    > coincidence, Brown really is in Barcelona. Again, Smith had a belief that
    > was true and justified, but not knowledge.

Or from 8th century Indian philosopher Dharmottara:

   > Imagine that we are seeking water on a hot day. We suddenly see water, or so we 
   > think. In fact, we are not seeing water but a mirage, but when we reach the 
   > spot, we are lucky and find water right there under a rock. Can we say that we 
   > had genuine knowledge of water? The answer seems to be negative, for we were 
   > just lucky. 

More to the point, the definition of knowledge as linguistic agreement is convincingly supported by much of what has historically been common knowledge, such as the meddling of deities in human affairs, or that the people of Springfield are eating the cats.

I don’t think it’s so clear cut… Even the most adamant “facts are immutable” person can agree that we’ve had trouble “fact checking” social media objectively. Fluoride is healthy, meta analysis of the facts reveals fluoride may be unhealthy. The truth of the matter is by and large what’s socially cohesive for doctors’ and dentists’ narrative, that “fluoride is fine any argument to the contrary—even the published meta-analysis—is politically motivated nonsense”.