Conversation

forum posts, usenet, bbs: user to board; board to everyone with access to the board, including non members

chat rooms, group chat, irc: user to chat, chat to everyone who is a member of chat and only them

direct messages: 1 to 1, user to user

twitter, facebook: disjoint sets: user posts, posts go to followers, each user has a different set of followers; posts may also be visible to everyone including non members.

here’s a random thought:
what if a social thing had spatial awareness?