1. A method for identifying documents referring to an entity, the entity being associated with a first set of features, the method comprising: at a computer having one or more processors and memory storing programs for execution by the one or more processors: identifying a first set of documents based on a first model and the first set of features, wherein the first model includes a first set of rules specifying at least one combination of features from the first set of features that are sufficient for identifying a document referring to the entity, and each document in the first set of documents includes a sufficient number of features in common with the first set of features to identify a document referring to the entity according to the first model; determining a second model based on features included in one or more documents in the first set of documents, wherein the second model includes a second set of rules specifying at least one combination of features from the first set of documents that are sufficient for identifying a document referring to the entity; identifying a second set of documents based on the second model, wherein each document in the second set of documents includes a sufficient number of features in common with the first set of features to identify a document referring to the entity according to the second model, and wherein the second set of documents includes at least one document not included in the first set of documents; and extracting one or more facts from the second set of documents and associating the extracted facts with the entity.
10. The method of claim 1 , further comprising: estimating importance of the entity based on a number of documents in the second set of documents.