Facebook Vanity URLs
If you’re up right now, don’t forget to take advantage of the opening of Facebook Vanity URLs which changes the address of your profile from something like this:
facebook.com/9581189
to this:
facebook.com/your.name
Not sure what to pick? Standard domain name rules apply. Pick something easy to remember, easy to spell and as short as possible. I tried for /guin but it turns out the username has to be at least five characters, so I managed to snag facebook.com/guinevere.
If you want your own vanity URL at Facebook, click here to get one now.

Mashable is reporting that over 1/2 million Facebook usernames were registered in the first 15 minutes:
http://mashable.com/2009/06/12/facebook-team-celebrates-username-launch/