We've been exploring the idea of machine-readable content licensing in Tent (https://tent.io). The idea being that you could subscribe to content feeds based on your willingness to accept certain terms of service.
For example, as a distributed system, there's no guarantee that your subscribers will delete posts you sent them when asked, so one content license might be that you agree in advance to delete any content published under this license within a specified number of hours.
We've also looking at the idea of community arbiters: if two users want to enter into a contract, they could choose an arbiter to help resolve any disagreements from the intersection of their social graphs.
Depending on the situation it might be very obvious (they republish it after the deletion request) or you might not know without examining their system directly.
There are a number of options depending on the context. If Tent ends up like email (90% of users spread across a handful of service providers), then one term of the license could be consent to regular audits by mutually trusted individuals or organizations (e.g. the EFF).
For example, as a distributed system, there's no guarantee that your subscribers will delete posts you sent them when asked, so one content license might be that you agree in advance to delete any content published under this license within a specified number of hours.
We've also looking at the idea of community arbiters: if two users want to enter into a contract, they could choose an arbiter to help resolve any disagreements from the intersection of their social graphs.