Wiki has taken much inspiration from Alexander who wrote that A City is not a Tree. Speaking to people who know that work I explain why this wiki is more aligned with ideals of Alexander and his colleagues than those embodied in popular social networks.
Although the federation relies on domain names which are organized as a tree, the interactions between federated sites overlays this network as if the network were completely flat.
There is no communication through a private database managed by a platform provider with policy and algorithms that serve their own purpose.
The exception regarding visibility would be those sites on a private network such as a corporate LAN. Authors on those sites will 'fork' their pages to public sites when they want to share their work under the creative-commons license that makes the "visible federation" whole.
Similarly, if an organization were to host public wiki sites for members, there would be some expectation of service which might be expressed in terms and binding in a court of law. The federation overlays these associations too. We provide export/import mechanisms as insurance that dissatisfied authors can take their content elsewhere.
Facebook, Twitter, Skype, Linked, Google Docs, and Github are not similarly open to organic reorganization in response to our real and cyber environments.
The Blogosphere was once federated until corporate interests offered a reader, captured the community and then withdrew the product in favor of their own private database solution.