10. An information retrieval system, comprising: one or more processing units; a microblog URL filter configured for pairing a query with a plurality of microblog resource identifiers, wherein each microblog resource identifier comprises a resource identifier obtained from monitoring a received data stream of microblog posts, thereby generating a plurality of query/microblog resource identifier pairs; a first feature generator, at least partially executed by at least one of the one or more processing units, configured for generating a feature set for each query/microblog resource identifier pair, wherein generating the feature set includes generating at least one textual feature by analyzing text of one or more microblog posts that refer to the microblog resource identifier in conjunction with text of the query or generating at least one social networking feature by analyzing one or more characteristics associated with one or more microblog users that issued or received the microblog resource identifier via the microblog; a first machine learned ranker configured for processing the feature sets associated with each query/microblog resource identifier pair to produce a ranking for each microblog resource identifier; and a ranked resource identifier combiner configured for generating one single combined ranked list of resources by 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, wherein each network resource identifier comprises a resource identifier obtained from resources other than the received data stream of microblog posts.
10. An information retrieval system, comprising: one or more processing units; a microblog URL filter configured for pairing a query with a plurality of microblog resource identifiers, wherein each microblog resource identifier comprises a resource identifier obtained from monitoring a received data stream of microblog posts, thereby generating a plurality of query/microblog resource identifier pairs; a first feature generator, at least partially executed by at least one of the one or more processing units, configured for generating a feature set for each query/microblog resource identifier pair, wherein generating the feature set includes generating at least one textual feature by analyzing text of one or more microblog posts that refer to the microblog resource identifier in conjunction with text of the query or generating at least one social networking feature by analyzing one or more characteristics associated with one or more microblog users that issued or received the microblog resource identifier via the microblog; a first machine learned ranker configured for processing the feature sets associated with each query/microblog resource identifier pair to produce a ranking for each microblog resource identifier; and a ranked resource identifier combiner configured for generating one single combined ranked list of resources by 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, wherein each network resource identifier comprises a resource identifier obtained from resources other than the received data stream of microblog posts. 14. The system of claim 10 , wherein the first feature generator generates a social networking feature for a query/microblog resource identifier pair by calculating a measure of authority of a microblog user that issued the microblog resource identifier via the microblog based on an analysis of follower and following relationships between microblog users.