I recently was asked what was the resource I showed in the ArcGIS Online Steps to Success session to monitor activity with your ArcGIS Online Organizational accounts. I showed the Activity Dashboard which can be found here: Activity Dashboard for ArcGIS . Register with your ArcGIS Online administrator account and you can access lots of detailed metrics about your site's users, usage, credits and more.
Here are recent Blog posts, too for more information about the tool: Activity Dashboard for ArcGIS — July Update | ArcGIS Blog and (older) Activity Dashboard for ArcGIS Now Available | ArcGIS Blog
Hope this helps!