The MindLink performance counter suite indicates general usage of the system including number of messages sent and received, joined groups etc.
Mobile, Anywhere, SharePoint
The ActiveSessions counter records the number of user sessions connected to the node. In Skype for Business terms, this is the number of SfB endpoints maintained by the server on behalf of logged on users.
For Mobile, this number represents the number of users who have a logged on server-side session, not necessarily users who are actively using the app in the foreground on their device. For Anywhere and SharePoint, this is the number of active browser sessions open at the current time.