1. An interoperability network configured to facilitate messaging and mediate policy differences among a plurality of independent services and users having associated client machines, the interoperability network comprising one or more computing devices configured to: receive a request for a first service to perform a particular task involving a second service on behalf of a first user, wherein the first and second services are in communication with an interoperability network and are provided by first and second independent service providers, respectively; determine whether the first user has provided a first set of credentials that defines access information for the second service, the first set of credentials being included among a plurality of sets of credentials stored on one or more storage media accessible through the interoperability network; determine whether the first service is authorized to act on behalf of the first user with respect to the second service with reference to one or more of a plurality of permissions stored on the one or more storage media; and where the first user has provided the first set of credentials, and where the first service is authorized to act on behalf of the first user with respect to the second service, authorizing the first service to perform the particular task involving the second service on behalf of the first user.