SharePoint Newsfeed used to be the default option for social experiences in Office 365. If you want to use the Newsfeed, you can turn it on in the SharePoint Online admin center
Newsfeed or Yammer?
Office 365 includes two options for enterprise social features: Yammer and the SharePoint Newsfeed. If you pick Yammer to be your social experience, follow the Enterprise Activation process to activate Yammer. After activation, a Yammer tile appears on the Office 365 portal page and the app launcher, making it easy for Office 365 users to reach Yammer. Also, if your network is eligible, users can sign-in to Yammer using their Office 365 account.