In psychology, the idea that everyone is like us is called the “false-consensus bias.”
https://en.wikipedia.org/wiki/False-consensus_effect
We tend to assume that most people think and feel similarly to us. We then categorize those who don't as "other" and somehow inferior. This tendency is intensified when we gather with people who are in fact like-minded. And that happens in social media, where we tend to follow those with similar views.