A good way to centralize content and interaction on a social network.
Updates, events, images, video, forum...
Facebook is a general network based on "friending" and sharing of messages and media
ning is a netowrk that is more specific to a particular topic, location or interest. It has blog, network, forum, and media sharing functionality, but is not particulary "contact" or "friending" based.
Plaxo is a network cenetered around contact info and aggregation of content
simple basic link stroing and sharing and hosting