1. A method comprising: storing, in a plurality of remote data hosts, entries of action data describing actions performed by one or more users of a social networking system, each remote data host storing a subset of entries of the action data; receiving a triggering event to modify an action cache; determining an engagement score for each of the entries of action data, the engagement score of an entry indicating a probability of a user interacting with a notification based on the entry; determining a plurality of proxy sets associated with a plurality of proxy data aggregators, a proxy set including entries of action data, each proxy set determined from engagement scores of entries of action data stored in one or more remote data hosts coupled to a proxy data aggregator; receiving the plurality of proxy sets at a social data aggregator; selecting entries of action data from the plurality of proxy sets based on the engagement scores associated with the entries of action data; and storing the selected entries of action data in the action cache.