1. A method of predicting geographic population density, comprising: defining a Markov Random Field (MRF) model for estimating a number of mobile devices used within a geographic area, wherein the MRF model represents a plurality of state variables x i as nodes in a regular 3-D lattice, wherein nodes in a plane of the lattice describe a plurality of corresponding geographic areas arranged in a grid, values of x i at the nodes in the plane indicate estimated numbers of mobile devices being used within the corresponding geographic areas at a time slice, and a third dimension of the lattice has nodes indicating values of x i at time slices preceding and following the time slice of the plane; applying current data describing a number of mobile devices currently observed within the geographic area and historical data describing a number of mobile devices historically observed within the geographic area to the MRF model to produce an estimate of a number of mobile devices currently being used within the geographic area; predicting using a processor a population density for the geographic area responsive to the estimated number of mobile devices currently being used within the geographic area; and storing the predicted population density for the geographic area in a computer-readable storage medium.