People were always like this. You just weren't able to see it, because you were in an offline "filter bubble" of peaceful and reasonable people. There have been countless civil wars in history after all.
Perhaps to some extent. There is always a baseline level of division and conflict. But I do believe there are things that can be done to amplify or suppress division in a society. I think we're leaning real hard on the amplification of division right now. None of this stuff has been around long enough to get an overall feeling on the effect it has on a society that has it versus a similar level society that does not, so it's hard to say for sure what the effect will be. I have a bad feeling about it though.
Yes because before the internet there was never division - like actual separate water fountains, door that you had to go in based on the color of your skin, schools, redlining, and racial covenants.