A customer recently asked me why their popularity trend reports were no longer showing any data in SharePoint. The customer had recently moved all of their search functionality to a hybrid cloud search service application from a traditional on-premises Search Service Application. All their search functionality is now powered by Office 365. As of today, popularity trend reports will not work with hybrid search. The following explanation was provided by Manas Biswas, hybrid search expert and co-author of the awesome Configuring Microsoft SharePoint Hybrid Capabilities. 🙂
Popularity trends works based our of analytics. Usage Analytics reporting isn’t operational under a Cloud SSA as of today. This is because the usage analysis uses the view events created on the content farm. Since the content farm is on-premise the view events are not passed on to the search farm that is in the cloud. The analysis is run on the search farm so it will not see the view events and have the possibility to update the usage reports.
If you'd like this functionality in the future, please please please, submit this feedback on uservoice.