  The Decentralised Web (DW) has recently seen a renewed momentum, with a
number of DW platforms like Mastodon, Peer-Tube, and Hubzilla gaining
increasing traction. These offer alternatives to traditional social networks
like Twitter, YouTube, and Facebook, by enabling the operation of web
infrastructure and services without centralised ownership or control. Although
their services differ greatly, modern DW platforms mostly rely on two key
innovations: first, their open source software allows anybody to setup
independent servers ("instances") that people can sign-up to and use within a
local community; and second, they build on top of federation protocols so that
instances can mesh together, in a peer-to-peer fashion, to offer a globally
integrated platform. In this paper, we present a measurement-driven exploration
of these two innovations, using a popular DW microblogging platform (Mastodon)
as a case study. We focus on identifying key challenges that might disrupt
continuing efforts to decentralise the web, and empirically highlight a number
of properties that are creating natural pressures towards recentralisation.
Finally, our measurements shed light on the behaviour of both administrators
(i.e., people setting up instances) and regular users who sign-up to the
platforms, also discussing a few techniques that may address some of the issues
observed.
