As a somewhat long-term net user, I can attest to the fact that they just evolve. Here, I use my real name and initial. On ICQ, I use one of my stage names. On other boards and on other services, the name varies. How do I keep 'em all straight? Hell if I know. I've stopped using a couple of services over the years, and if pressed, I probably couldn't tell you what my codes were. I probably have a few I've never even activated past a first drunken "Hey, this looks cool! Let's call myself garhawo;eiru this time!".
My theory is that if the site/service/account is important enough, you'll remember whatever you call yourself. And if not, write it down someplace.