Usage

The Usage reports show an overview of the usage of some of the most important online components of Office 365 (such as Exchange, OneDrive, SharePoint, and so on) for 7 days, 30 days, 90 days, or 180 days, followed by detailed bar charts of various types of activities within those components (for example, the charts for Exchange track messages sent, received, and read, separately):

Usage reports page

These are the effective ways to track user adoption and see if there are any issues that need addressing. (If there's very little content on your company's main SharePoint team site, and yet usage and the number of files are unusually high, there may be a site collection that someone else created and is using that isn't being tracked by your centralized governance, for instance.)