When activating the Yammer Securlet an error is displayed and the securlet is not activated. “Securlet is not activated, because the authorizing user does not exist in CloudSOC."
The primary email address of the Yammer account being used to activate the securlet did not match the Username of the CloudSOC user.
To resolve this issue look at the email aliases of the Yammer Admin user and verify that the primary email address matches the CloudSOC SysAdmin user that is logged into the CloudSOC tenant when doing the activation. In the case of Yammer and Office 365 the offending email is likely the one that has the domain.onmicrosoft.com as the email domain.