How many different identities can one person sanely manage?

...asks John Udell, in Critical mass and social network fatigue.  I'd argue that the answer is about one and a half, in the long run.  (The Shockwave Rider pushed it a lot further, but then he was founding cyberpunk.  Also, he was fictional.)

When "identity" was just basically a system-local nickname used between you and a service, having multiple names for yourself was a minor inconvenience at worst, and sometimes mildly useful for privacy.  Now that the services are more and more intermediaries between people, nicknames are a problem.  Jon notes:
Years ago at BYTE Magazine my friend Ben Smith, who was a Unixgreybeard even then (now he’s a Unix whitebeard), made a memorablecomment that’s always stuck with me. We were in the midst of evaluatinga batch of LAN email products. “One of these days,” Ben said in, Ithink, 1991, “everyone’s going to look up from their little islands ofLAN email and see this giant mothership hovering overhead called theInternet.”
Yep.  Email is still the killer app of the Internet, and it wouldn't be if it were still stuck in LAN mail silos.  Though spam is now so bad that I literally can't send email to a few people who can't manage their spam filters.  That's a real issue and represents a real opportunity as well.

While OpenID isn't a total solution for these issues, it's a critical piece of infrastructure.  As an example of the kinds of things it enables, take a look at Jyte.