9. A computer system comprising: a processor; a memory operatively coupled to the processor; and a program module (i) which executes in the processor from the memory and (ii) which, when executed by the processor, causes the computer system to rank documents with social tags by: crawling a user profile database to collect the social tags associated with a document, the social tags comprising a text string containing one or more terms at least one of describing, categorizing and providing information regarding the document, and being indexed and stored in user profile tables separately from the document they are associated with; determining, based on the social tags, a number ranking feature containing a number of times the document was tagged with the social tags; determining, based on the social tags, a textual property ranking textual stream containing a union of all the social tags associated with the document; transforming the number ranking feature into a static input value by applying a normalization constant to the number ranking feature; transforming the textual property ranking textual stream into a dynamic query dependent input value, wherein dynamic query dependent input value includes a number of times a given term in a query appears in the textual property ranking textual stream; and determining a document rank for the document by inputting the static input value and the dynamic query dependent input value into a ranking function.
13. The computer system of claim 9 , wherein transforming the textual property ranking textual stream into a dynamic query dependent input value comprises inputting textual property information associated with the textual property ranking textual stream into a Okapi Simple Best Match 25F (BM25F) ranking function to generate the dynamic query dependent input value.