Bad thing, in the user login report, you cannot identified which login is coming from Salesforce1. Work around, you can download Login History into CSV file and using Ms Excel to filter Application contain Salesforce1. It is pretty complex using Login History and you cannot run it as a report to use it as source for Dashboard and present it your management team.
Instead of using standard User report type, create a new custom report. Here is the step:
Create Custom Report Type
Start with create a new custom report type:
• From Setup, click Create | Report Types
• Click New Custom Report Type button
• Enter the following values.
• Primary Object: Users
• Report Type Label: a unique label such as Login History with Identity
• Report Type Name: this field will be automatically populated based on Label, just skip it.
• Description: Give a useful description that other user will understand in the future.
• Store in Category: Pick a category for this report; such as Administrative Reports.
• Deployment Status: Keep as In Development until ready to deploy this report for other users.
• Click Next
• Select Click to relate to another object
• Select Identity Event Logs (Users)
• Keep A to B Relationship to default "Each "A" record must have at least one related "B" record"
• Click Save.
Create Custom Report• Click the Reports tab and New Report button
• In Administrative Reports, select Login History with Identity and click Create.
• Drag-and-drop fields on to the report. Some useful fields to add into report: Full Name, App: Connected App Name, Timestamp, Status and Usage Type
• Add filter “App: Connected App Name contains Salesforce1”
• Give the report a name, such as: Salesforce1 Adoption.
• Click Save
As this is a normal report, you can adjust the Time Frame.
Here is the screenshot of the report created:
If you see above report, in Usage Type:
• OAuth Authorization: when the user first logs in
• OAuth Token Exchange: when user has been idle for some time and the app needs to re-authenticate
In the Status, Error: App access denied tell you if user hit "Deny" in Salesforce1 app
If you need to do further analysis, you can download the CSV file and use Pivot Table in Ms Excel.
Anyway, this only work in Enterprise edition and above (included Unlimited and Performance edition), so if you are using Professional edition, this is not applicable and hope Salesforce will implement Salesforce Identity for Professional edition in the future.