In a space like twitter, you don't know who is in the room with you. You have your "regulars", and those are who you keep in mind when avoiding inflicting harm, but retweets and quote tweets and screencaps mean that your words could reach literally anybody.
@darius also has a Mastodon fork which adds the concept of "local-only" posts which can't be boosted outside the community. This blurs the line between chatroom and microblog in an interesting way!
The social network of the future: No ads, no corporate surveillance, ethical design, and decentralization! Own your data with Mastodon!