This is something that a new data structure could help with. I’ve written down what I was thinking of here for reference:
At the moment we could still implement this for core auth providers - just read the user_ids from {google,twitter,facebook,...}_user_info tables. Plugins tend to use oauth2_user_infos, so that is achievable. Plugins that use their own data format in PluginStoreRows will be pretty much impossible to provide statistics for with the current structure.