Salesforce has started to expose information relating to the usage of your package components in the subscriber orgs since the Spring '14 release of the platform. This enables you to report what Custom Objects and Visualforce pages your customers are using and more importantly those they are not. This information is provided by Salesforce and cannot be opted out by the customer.

At the time of writing, this facility is in pilot and needs to be enabled by Salesforce Support. Once enabled, the MetricsDataFile object is available in your production org and will receive a data file periodically that contains the metrics records. The Usage Metrics Visualization application can be found by searching on AppExchange and can help with visualizing this information.