Here are some of the user account servers I'd love to see come to the fediverse:
- Apple ID. 1.5B users. Already used for an ID, but not a social one.
- Firefox ID. 300M? users. Mozilla is an ally that has announced future support of ActivityPub.
- Google ID. 1.8B users. Widely used. Many social products, but no general-purpose one.
- Microsoft ID. 1B? users. Used mostly for Office 365, Teams, and XBox. Possible connection through Teams.