1. A computer-implemented method for defining a segment based on interaction proneness, the method comprising: receiving online activity data that specifies instances of presentation for one or more content items, and instances of user interaction detected for any of the content items; training at least one predictive model on the online activity data, the predictive model trained to predict interaction proneness based on a cross product of two characteristics associated with the instances of user interaction; identifying, using the predictive model, the cross product as being associated with the interaction proneness; and generating at least one segment definition that takes into account the identified cross product.