1. A system comprising: computer-readable media; one or more processors; and one or more modules stored in the computer-readable media and executable by the one or more processors to perform operations comprising: accessing a multimedia data item; extracting features from the multimedia data item; applying a classifier to the features to determine similarity values corresponding to individual labels of a plurality of labels, the classifier including a plurality of model vectors each corresponding to one of the individual labels; determining whether the multimedia data item is a positive multimedia data item or a negative multimedia data item; and updating at least one model vector of the plurality of model vectors, wherein updating the at least one model vector comprises applying a first update for the positive multimedia data item and a second update for the negative multimedia data item.