As part of the new Client Side Object Model (CSOM) version (4622.1208 or newer), SharePoint has new capability for bulk importing custom user profile properties. This capability has been now enabled in the first release tenants in Office 365 cross the world. Previously you could have taken advantage of the user profile CSOM operations for updating specific properties for user profiles, but this is not that performant and in case of thousands of profiles, operation is too time consuming.
http://dev.office.com/blogs/introducing-bulk-upa-custom-profile-properties-update-api
Archives For March 31, 2016
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