Establishing Global Policies over Decentralized Online Social Networks
  Conventional online social networks (OSNs) are implemented in a centralized manner. Although centralization is a convenient way for implementing OSNs, it has several well known drawbacks. Chief among them are the risks they pose to the security and privacy of the information maintained by the OSN; and the loss of control over the information contributed by individual members.   These concerns prompted several attempts to create decentralized OSNs, or DOSNs. The basic idea underlying these attempts, is that each member of a social network keeps its data under its own control, instead of surrendering it to a central host; providing access to it to other members of the OSN according to its own access-control policy. Unfortunately all existing DOSN projects have a very serious limitation. Namely, they are unable to subject the membership of a DOSN, and the interaction between its members, to any global policy.   We adopt the decentralization idea underlying DOSNs, complementing it with a means for specifying and enforcing a wide range of policies over the membership of a social community, and over the interaction between its disparate distributed members. And we do so in a scalable fashion. 
Zhe Wang, Naftaly H. Minsky
cs.SI