  Online social networks, such as Facebook and twitter, are a growing
phenomenon in today's world, with various platforms providing capabilities for
individuals to collaborate through messaging and chatting as well as sharing of
content such as videos and photos. Most, if not all, of these platforms are
based on centralized computing systems, meaning that the control and management
of the systems lies in the hand of one provider, which must be trusted to treat
the data and communication traces securely. While users aim for privacy and
data sovereignty, often the providers aim to monetize the data they store.
Even, federated privately run social networks require a few enthusiasts that
serve the community and have, through that, access to the data they manage. As
a zero-trust alternative, peer-to-peer (P2P) technologies promise networks that
are self organizing and secure-by-design, in which the final data sovereignty
lies at the corresponding user. Such networks support end-to-end communication,
uncompromising access control, anonymity and resilience against censorship and
massive data leaks through misused trust. The goals of this survey are
three-fold. Firstly, the survey elaborates the properties of P2P-based online
social networks and defines the requirements for such (zero-trust) platforms.
Secondly, it elaborates on the building blocks for P2P frameworks that allow
the creation of such sophisticated and demanding applications, such as
user/identity management, reliable data storage, secure communication, access
control and general-purpose extensibility, features that are not addressed in
other P2P surveys. As a third point, it gives an overview of proposed P2P-based
online social network applications, frameworks and architectures. In specific,
it explores the technical details, inter-dependencies and maturity of the
available solutions.
