Generally speaking, plaza are public while warrens are private. Plaza are easy to expand, because people can see what is going on in the community and decide whether to join the community. On the contrary, warrens are personalized contents in social network, which makes they scale free. Therefore, communities that have a plaza-like structure are easy to expand, thus suffering more from Evaporative Cooling Effect, while communities having warren-like structure are not very scalable, but more stable. A successful social network should somehow combining those two structures, taking both scalability and stability into account.
IndieWeb has both a big and expandable plaza space (the wiki and commons spaces) as well as warrens (individual sites interacting with each other separate from the main plaza).