1. A method for generating item recommendations, the method comprising: for each user of a plurality of users, obtaining one or more measures of affinity between the user and specific items of a plurality of items represented in an electronic catalog; generating a user-item association model in a computer memory based, at least in part, on probabilistic matrix factorization, wherein the user-item association model associates a user account with a plurality of personas in accordance with a persona distribution, each persona corresponding to a different respective group of catalog items; optimizing the user-item association model based, at least in part, on computer generated randomization, wherein the computer generated randomization is constrained by the obtained affinity measures; and during a browsing session of a target user: recording interactions of the target user with the electronic catalog; associating the browsing session with the user account; determining an active persona for the browsing session based, at least in part, on the optimized user-item association model and the interactions recorded during the browsing session, wherein the active persona is one of the plurality of personas; generating persona-specific recommendations of catalog items for the target user based on item preferences associated with the active persona; and generating, and outputting for presentation to the target user, a user interface that displays the persona-specific recommendations; the method performed programmatically by one or more computing systems under control of executable program code.