Hourly Active Users (HAU) is a metric that measures the number of unique users who have engaged with a startup's product or service within a given hour.

Snapchat most famously popularized HAU as it achieved product-market fit and saw hockey stick growth.

It is an even more granular measure of user engagement than Daily Active Users (DAU) and provides a real-time view of user activity.

Hourly active users can be useful for startups with a highly dynamic user base and require a more detailed understanding of user behavior.

For example, a real-time chat app or a gaming platform might report its hourly active users as a way of demonstrating the level of engagement of its users with the product in real-time.

A high number of hourly active users can indicate strong user engagement and a growing customer base, while a decline in hourly active users can indicate that the startup is struggling to retain its users or that there are issues with the product or service that need to be addressed.