A system comprising at least one server computer having at least one processor programmed to: receive, over a network, activity data of a first user measured by an activity monitoring device; process the activity data of the first user to update a value of an activity metric for the first user; responsive to updating the value of the activity metric for the first user, identify a change in an inequality relationship between the value of the activity metric for the first user and a value of the activity metric for a second user; and, responsive to identifying the change in the inequality relationship, trigger, over the network, a first user device to prompt the first user to generate a message to the second user for display on a second user device.