Are you missing the activity feeds in your CRM system?
Recently I have come across a problem, that the activity feeds feature was not available in the CRM 2015.
After some investigation, we realized that it was due to the fact, that someone turned on the integration with Yammer.
The Yammer is not used, but we cannot go back to the Activity feeds now, so please be aware of this! Once you enable Yammer integration, you lose the Activity feeds.