Websites like myspace and facebook establish relationships between distinct users using the "friending" ideology, where one user sends a request to be accepted by another user in order for them to have the mutual permission to do stuff like post messages on each others walls.

Less restrictive than the "friending" ideology, Twitter and instagram use the followers/following ideology where you can subscribe to the tweets or posts of another user without their permission.

Less restrictive than the "followers/following" ideology, email and calling someone on the phone allows you to directly contact anyone.

Are there other ideologies that have been successfully implemented either in social networking sites or other real world constructs to establish relations between users?