Here's a side by side feature comparison between Yammer and Bitrix24.
* Please note that the information may have changed since the publication. For current prices and features please visit Bitrix24 pricing page.
Easy setup and onboarding
Integration with many Microsoft products
Large online community
Windows Phone support
Too social, not enough worktools
Low adoption rates ("We have Yammer, nobody uses it")
No on-premise version you can host on your own server
Few customization options
Must pay for SharePoint, Office 365, Lync, Exchange, OneDrive to make it 'usable'