8. The method of claim 1 , wherein combining the rankings for each microblog resource identifier produced by the first machine learned ranker with rankings generated for a plurality of network resource identifiers comprises: pairing the query with the plurality of network resource identifiers, thereby generating a plurality of query/network resource identifier pairs; generating a feature set for each query/network resource identifier pair; and processing the feature sets associated with each query/network resource identifier pair in a second machine learned ranker to produce the ranking for each network resource identifier.
8. The method of claim 1 , wherein combining the rankings for each microblog resource identifier produced by the first machine learned ranker with rankings generated for a plurality of network resource identifiers comprises: pairing the query with the plurality of network resource identifiers, thereby generating a plurality of query/network resource identifier pairs; generating a feature set for each query/network resource identifier pair; and processing the feature sets associated with each query/network resource identifier pair in a second machine learned ranker to produce the ranking for each network resource identifier. 9. The method of claim 8 , wherein generating the feature set for each query/network resource identifier pair comprises: 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.