I know this doesn't add much value to the discussion, but I was really proud of my UIN when I was a teenager. And this may be my last chance to flaunt it, so here it is:

1779900

So back in the day, these were known as Universal Internet Numbers, or UINs. You have to admire the sheer audacity of using that name for the user identifiers of a service you're building. I believe they were renamed to "ICQ#" later.