I think the idea of social media dividing us ignores the scale of it. All those other examples you gave were very local, and in that environment a consensus can form about certain political or ideological views. Those views could be vastly different than those a similar sized community holds 100 miles away though. Social media and it's global scale exposes those differences and makes consensus on any sort of issue impossible.

At the same time it also allows for minority solidarity outside of the traditional local community. For example there may only be 1 or 2 LGBT+ people in a town, which can easily be marginalized, shamed and ignored. But if they're able to communicate across geographic boundaries they're able to create a larger stronger community that is harder to ignore. It also does the same for nazis though.