Exploiting temporal influence in online recommendation

In this paper we give methods for time-aware music recommendation in a social media service with the potential of exploiting immediate temporal influences between users. We consider events when a user listens to an artist the first time and this event follows some friend listening to the same artist short time before. We train a blend of matrix factorization methods that model the relation of the influencer, the influenced and the artist, both the individual factor decompositions and their weight learned by variants of stochastic gradient descent (SGD). Special care is taken since events of influence form a subset of the positive implicit feedback data and hence we have to cope with two different definitions of the positive and negative implicit training data. In addition, in the time-aware setting we have to use online learning and evaluation methods. While SGD can easily be trained online, evaluation is cumbersome by traditional measures since we will have potentially different top recommendations at different times. Our experiments are carried over the two-year "scrobble" history of 70,000 Last.fm users and show a 5% increase in recommendation quality by predicting temporal influences.