In Cognos Reporting there is a folder labeled Mobile Usage Metrics. Here you could add Employee and Device Platform. This should show all employees using the Mobile app. Since ExpenseIt is now built into Concur mobile, the count would be the same.
Also you can use Cognos to see if an expense entry was created by ExpenseIt. The path to the data item is: Expense Reports > Entry Information > Additional Entry Details. The item is called "Is Created by ExpenseIt". The values should be either Y or N.