1. A method of notifying a target user of an event, the method comprising: receiving, by a server system, event data, the event data identifying a target user; generating, by the server system, at least a first notification and a second notification for the target user in response to the event data; sending, by the server system, the first notification to the target user via a first notification channel; queuing, by the server system, the second notification in a queue, the queuing being performed at a start time; determining, by the server system, whether a hold time has elapsed since the start time; after the hold time has elapsed, determining, by the server system, whether the target user has acted on the first notification; in the event that the target user has not acted on the first notification, sending the second notification to the target user via a second notification channel, the second notification channel being different from the first notification channel; and in the event that the target user has acted on the first notification, removing the second notification from the queue.