1. A method for estimating advertisement inventory, comprising: clustering, using a processor, a plurality of viewed pages into a plurality of clusters to generate a plurality of page view probability vectors that includes a page view probability vector for each viewed page, said clustering based on a frequency of search keywords present in the viewed pages; clustering by the processor, a plurality of search keywords into the plurality of clusters to generate a plurality of keyword probability vectors that includes a keyword probability vector for each search keyword, the search keywords based on search keywords previously submitted by a plurality of users, and the plurality of viewed pages including previously viewed pages corresponding to the previously submitted search keywords; clustering the plurality of viewed pages into a plurality of clusters based on a time-decaying weighted frequency of the search keywords and an intensity value of the search keywords, present in the viewed pages; and estimating by the processor, a plurality of total inventories that includes an estimated total inventory for each cluster of the plurality of clusters.