It isn't actually a contraction because people create groups or join groups based on a real relationship whether a common interest, common event, or common context. Each group is silo'd from the others and if you aren't in any other groups you won't know about them.

The connections between groups only happens when people you get to know in one group start to build a real relationship that expands outside of that current group's context and lead to the creation of another group whether for another shared interest, or temporary event.

As people create more and more of these groups and these "mini-Facebooks" start to overlap users a new social graph starts to take shape not based on a 1 to 1 friend request but on real relationships, topics, and events.