In one embodiment, a method includes receiving first network usage data for user devices indicating activity by the user devices on webpages, determining first-level cluster centroids based on the first network usage data, and assigning each user device to at least one first-level cluster centroid. The method also includes determining second-level cluster centroids based on the first network usage data for user devices assigned to a particular first-level cluster centroid, and assigning each user device previously assigned to the particular first-level cluster centroid to at least one second-level cluster centroid. The method further includes determining clusters based on the first-level cluster centroids and the second-level cluster centroids, assigning each user device to at least one cluster, and sending one or more first notifications to user devices associated with a particular cluster.