1. A method of identifying user demographic characteristics, comprising: at a computing device with one or more processors, and memory storing one or more programs for execution by the one or more processors: acquiring a plurality of photos posted to a social media site by a user; identifying a plurality of facial images in the acquired photos; inferring that each of a subset of the plurality of photos is a respective self-potrait taken by a respective person corresponding to a respective facial image in the respective photo, wherein each inference is based on a respective image of the respective person's arm along the perimeter of the respective photo; selecting a subset of facial images from the plurality of facial images, including selecting one or more of the facial images based on their presence in one or more of the inferred self-portraits, wherein selecting the subset of facial images comprises: applying a clustering algorithm to group the plurality of facial images into a plurality of clusters; and selecting a cluster of the plurality of cluster that has a highest cluster purity; wherein the selected subset of facial images comprises the facial images in the selected cluster; and estimating one or more demographic characteristics of the user based on the selected subset of facial images.