The best replacement for slack is hipchat.

The best alternatives to slack are rated as following:
★★★★★  hipchat
★★★★  free slack
★★★★  float
★★  flowdock
★★  basecamp
★★  yammer
★★  total slack
★  flack
★  not to slack
★  lag
  campfire
  convo
  file slack
  total float
  slack

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Google Trends

Scale is based on the average worldwide traffic of slack in all years. Click on graph for more.
slack
hipchat
free slack