Last year the EFF published this "legal primer" for people hosting Fediverse instances

Two important notes: (1) This guide is focused on legal risks that flow from hosting other people’s content, under U.S. law. In general, the safe harbors and immunities discussed below will not protect you if you are directly infringing copyright or defaming someone. (2) Many of us at EFF are lawyers, but we are not YOUR lawyers. This guide is intended to offer a high-level overview of U.S. law and should not be taken as legal advice specific to your particular situation.