1. A system, comprising: a processing subsystem; and a memory subsystem storing instructions that causes the processing subsystem to perform operations comprising: accessing, for a plurality of first user devices, user activity data describing user activities for active applications performed by a user on the first user device during a first time period; accessing data describing presentation of one or more content items during the first time period on second user devices, each second user device being different from each first user device; for each first user device determining, based on the user activity data, for each of a plurality of second time periods that are sub-periods of the first time period, respective first levels of user engagement with the first user device for each second time period, wherein first plurality of respective first levels of user engagement indicates engagement with the first user device and at least a second plurality of respective first levels of user engagement indicates disengagement with the first user device; and determining, based on the user activity data, for each of the plurality of second time periods that are sub-periods of the first time period, respective second levels of user engagement with the one or more content items presented during the first time period, each determination based on a user model that models user engagement for a plurality of active applications and a plurality of content items, and wherein for at least one active application according to the user model: a first level of user engagement indicating engagement with the first user device during a second time period is indicative of engagement with a content item presented on the second user device; and a first level of user engagement indicating engagement with the first user device during another second time period is indicative of disengagement with the content item presented on the second user device.