Every customer is unique to us!
In the Workflow Summary Dashboard, you can find a widget for Sum of Unique Customers. It will show you a sum of all customers the agents were interacting with within the time frame.
Careful with the counting though!
One customer can interact with you across multiple channels within the given time frame. That means that even though the sum for FB and TW will show you 1 unique customer each.
However, if you filter both FB and TW, there will be only 1 unique customer ;)