WCF / ASMX :: Singleton Class Variables Being Reset

I have set the instanceContextMode to single to create a singleton class.I use the join to add a user to the list, and getuserlist() to retrieve the users. I added a service reference opened two browsers , made sure i simulated it such that i am adding two seperate users , and display the users returned by getuserlist. But its always displaying only one user , the current user. I also have a counter

private static int counter = 1
public int returnCounter()
{
return counter++;
}

This is returning incremented values when i open it in different browsers , but the list is always getting overritten. I am running this as a managed windows service, wsHttpBinding , and my client is a website accessing this service.

[ServiceBehavior(InstanceContextMode = InstanceContextMode.Single, ConcurrencyMode = ConcurrencyMode.Multiple)]