You won't have exact way to determine how many users are logged into chatter at a particular instance, but you can get close by having a functionality of Users logged into Chatter in last 5 mins or any other time frame using LastLoginDate DateTime field on User Object.
You won't have exact way to determine how many users are logged into chatter at a particular instance, but you can get close by having a functionality of Users logged into Chatter in last 5 mins or any other time frame using LastLoginDate DateTime field on User Object.