The best replacement for yammer is chatter.

The best alternatives to yammer are rated as following:
★★★★★  chatter
★★★  sharepoint
★★  lync
★★  jive
★★  slack
★  convo
★  linkedin
★  asana
★  hipchat
★  sharepoint newsfeed
★  podio
★  newsgator
★  yammer now
★  present.ly
  salesforce chatter
  tibbr
  sharepoint 2013
  google+
  socialcast
  mangoapps
  news gator

Your alternative for yammer is not listed? Suggest!
Speaking about yammer, one probably shouldn't forget about sharepoint.

Other search terms related to yammer and their ratings are:
★★★★★ sharepoint
★★ sharepoint 2013
You have in mind a companion for yammer and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of yammer in all years. Click on graph for more.
yammer
chatter
sharepoint