17. The system of claim 10 , further comprising: a network resource identifier filter that pairs the query with the plurality of network resource identifiers, thereby generating a plurality of query/network resource identifier pairs; a second feature generator that generates a feature set for each query/network resource identifier pair; and a second machine learned ranker that processes the feature sets associated with each query/network resource identifier pair to produce the ranking for each network resource identifier.
17. The system of claim 10 , further comprising: a network resource identifier filter that pairs the query with the plurality of network resource identifiers, thereby generating a plurality of query/network resource identifier pairs; a second feature generator that generates a feature set for each query/network resource identifier pair; and a second machine learned ranker that processes the feature sets associated with each query/network resource identifier pair to produce the ranking for each network resource identifier. 18. The system of claim 17 , wherein the second feature generator generates the feature set for each query/network resource identifier pair by generating at least one content feature based on analyzing content of a resource identified by the network resource identifier in conjunction with the text of the query and generating at least one aggregate feature based on analyzing popularity and/or usage characteristics of the resource identified by the network resource identifier.