Explanation/Reference:
Note:
* You can replace the Newsfeed link with a Yammer link on the top navigation bar for SharePoint. This functionality is included in Service Pack 1 (SP1) for SharePoint Server.
* The new Yammer app for SharePoint lets you embed Yammer feeds into on-premises SharePoint Server
2013 sites to make them more social and engaging. Before you can do that, you have to do the following steps:
Remove the Newsfeed Web Parts from My Sites and Team Sites.
Hide the user interface controls that provide social features.
Install the Yammer app for SharePoint.
Add the Yammer feeds to your sites.
* By using directory synchronization, your organization can use existing on-premises user accounts. Your organization can also significantly reduce operational costs and give its employees safer and easier access to Yammer.
Reference: Hide SharePoint Server 2013 social features
https://technet.microsoft.com/en-us/library/dn659293.aspx
Integrate Yammer with on-premises SharePoint 2013 environments
https://technet.microsoft.com/en-us/library/dn270535.aspx