Please let me know if there is a means of evaluating whether an installed plugin is being used or not or if there is a means of gathering usage statistics for a particular plugin.

To check what plugins are currently installed on the system, you can go to Admin>Plugins and see the list of them there. it is difficult to distinguish whether actual plugin is in use. There can be several plugins installed, yet not necessarily used throughout the application. It would be wonderful though to get such info and I'd thus strongly suggest for you to create a new feature request in our jira system. http://jira.atlassian.com - I'd like to leave the credit of this great idea for you - hence am requesting for you to create the ticket (otherwise am happy to do so)

Potentially, one could write some sort of a plugin that would be querying the DB - PLUGIN table and PLUGIN_DATA table and somehow then traversing through the content of all confluence pages and analyse whether a particular plugin is in use or not. Would create extra load on the app, and the dynamic info gathering process may slow down the instance. (perhaps there is much much better way of doing so) You may also ask the dev forums forums.atlassian.com