InsideFacebook.com: “Will Facebook Give Away Vanity URLs in a Landrush?“:
Under such a system, users would be able to get URLs like facebook.com/mark for their own personal profiles. (However, some of the most common first names are already taken.)
Administering such a landrush would be a significant challenge. Presumably, Facebook wouldn’t let users register trademarked terms or generic words like “lasvegashotels” that could be used to simply generate spammy SEO traffic. Facebook would have to establish clear rules for how the vanity URLs would be distributed.