Along the lines of birth names, think about cell phone numbers. To an extent, they can be chosen, but still largely they are arbitrary, and unlike home phone numbers, are not likely to be changed in a year or two. Your cell number will become part of your permanent identity, like your social security number.

Now screennames and email addresses, those are chosen, but back in the day when people first started chosing them, it was thought of as fun, arbitrary, random, unimportant. But when I chose my email address 10 years ago, how was I to know that it would become a permanent part of my identity, a representation of me, the way people come to know me? In a way, it has become my name to those people out there who I only communicate with through email. I may never change my email address or my screenname again, and every day that passes without them ever changing they become a more permanent lasting part of my identification in my life.