The population density for a geographic area is predicted using a Markov Random Field (MRF) model. A MRF model is defined for estimating a number of mobile devices being used within a geographic area. The MRF model includes a set of rules describing how to use current data describing mobile devices currently observed in the area, and historical data describing mobile devices historically observed in the area to produce the estimate. Values of weight parameters in the MRF model are learned using the historical data. The current and historical data are applied to the MRF model having the learned weight parameters, and cost minimization is used to estimate of the number of mobile devices currently being used within the area. This estimate is used to predict the population density for the area. The predicted population density can then be used to provide location-based services.