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Sure, many other sites let you do this. But the more interesting part is that Facebook has been promoting usernames for months, having let people get usernames in vanity URLs earlier this year. For example, my personal profile is now at www.facebook.com/EricEldon. Now, I can log in as EricEldon. The significance, or what there is of it, is that Facebook wants you thinking about using the site on its terms, rather than, say, in terms of the Gmail or Yahoo account you’ve been logging in through in the past.
source: insidefacebook.com