Google Apps added a new service "activity graphs", so admins can see at-a-glance how their users are accessing Google Apps. This information can help customers evaluate the success of Apps deployments and scale internal support resources efficiently.
"The activity graphs display info about the number of active users in your organization as measured by login activity. We display trend lines showing user activity for the prior 1-, 7- and 30-day periods for Gmail, Google Docs, Google Calendar, and Google Sites."
To view graphs, click "View Usage and Reports" under the "Advanced Tools" tab of Google Apps control panel. You can also check out Reporting API for even more granular data on Apps activity within your organization.