Chris has a interesting analysis of a recent move by Facebook to support username urls:
Arrington has a post that claims that "Facebook is getting wise to something MySpace has known from the start - users love vanity URLs."
I don’t buy it. In fact, I’m pretty sure that the omission of vanity URLs on Facebook is an intentional design decision from the beginning, and one that I’ve learned to appreciate over time.
I think this change is another result of Facebook's apparent fear and envy of twitter's recent popularity. However, I believe they fullfill different social functions, and that Facebook's attempts at competing in Twitter's function only weakens it's existing position.