Historically, lack of a shared reality has been the rule. The idea that we all inhabit one reality is a construct of modernity, with the rise of mass production, mass media, and mass culture. Before the printing press there was no way for ideas to spread far enough to create a shared reality across a whole nation; indeed, the concept of a nation dates from this time. The closest you got was religion, but religions had a knack for splitting into sects and then bitterly fighting each other - witness the Protestant Reformation and the Wars of Religion that followed. Those wars were specifically fought over whose version of reality (both of which seem very quaint today) was true: each believed that the other was an existential threat to eternity.

The Internet just ushered in post-modernism on a global scale, which undoes a lot of the thought-unification (some would say thought-control) that came with modernity.