Commenter PHB at Crooked Timber:

There are many different definitions of identity, not all of which make sense. I prefer the view that an identity is a set of assertions about yourself that you may lay claim to. So in a sense everyone only has one identity and has only ever had one ‘identity’. But in practice we expose different sets of claims depending on the circumstances. Nobody puts their membership in Alcoholics Anonymous on their CV.

In response to Mark Zuckerberg, founder of Facebook:

“You have one identity,” he emphasized three times in a single interview with David Kirkpatrick in his book, “The Facebook Effect.” “The days of you having a different image for your work friends or co-workers and for the other people you know are probably coming to an end pretty quickly.” He adds: “Having two identities for yourself is an example of a lack of integrity.”

Huh. I’m concerned, naturally.