1. A system, comprising: a network; and a plurality of peer nodes coupled to the network and configured to: receive from a node on the network a request for notification of registration of a user identifier in a distributed index, wherein the request for notification comprises the user identifier, and wherein each of the plurality of peer nodes stores a zone of the distributed index configured to store presence information for a subset of user identifiers at locations in the zone indicated by hashes of the subset of user identifiers; store the request for notification of registration of the user identifier at a location in the distributed index indicated by a hash of the user identifier comprised in the request for notification; receive a registration request comprising the user identifier; access the stored request for notification of registration of the user identifier from the distributed index at the location indicated by a hash of the user identifier comprised in the registration request; and notify the node that sent the request for notification of registration of the user identifier of the registration of the user identifier in response to the request for notification of registration of the user identifier.