I think the really crazy thing about the online world is the separation of all the different spaces and the alienation that comes as a result.
In the real world, it's physical space. You gotta go through the city to get anywhere, see other people and see the world (even if it's only a tiny slice of it). The people around you necessarily have similar experiences because you're in the same physical space.
Online, though, everyone can be isolated into a completely abstract Skinner box of dopamine shots administered at the frequency of their choosing. Your experience is your own, and only your own, with interactions with other people made as atomizing as possible by various mechanisms. People have to be consciously reminded that the people they talk with online are real humans behind a screen, because otherwise it really is just a sea of usernames vomiting text and media.
The online world is so effective at atomizing its users that they forget there even is a society, it just becomes niche subcultures where other individuals are barely even perceived.