Resource optimization for online services is described. In one example, objects (such as mailboxes or other data associated with an online service) are assigned to network elements (such as servers) by inferring a relationship graph from log data relating to usage of the online service. The graph has a node for each object, and connections between each pair of objects having data items in common. Each connection has a weight relating to the number of common data items. The graph is partitioned into a set of clusters, such that each cluster has nodes joined by connections with a high weight relative to the weight of connections between nodes in different clusters. The objects are then distributed to the network elements such that objects corresponding to nodes in the same cluster are located on the same network element.