1. A method for analyzing user engagement comprising: associating multiple user activities with a plurality of behavior classes, wherein a user activity is associated with one or more behavior classes; obtaining event records for the multiple user activities; quantizing time to a temporal resolution, resulting in a plurality of quantized times; aggregating the event records, for each of the plurality of quantized times, for a behavior class of the plurality of behavior classes, wherein the aggregating comprises: identifying, for the quantized time, event records for activity types associated with the behavior class, and computing a count of the identified event records for activity types associated with the behavior class for the quantized time; accumulating the computed counts of event records for activity types associated with the behavior class for each quantized time; and computing percentiles for the accumulated counts of event records for activity types associated with the behavior class for each quantized time.