The vanity URL is an important aspect of most social media websites. Being able to choose the URL for your social profile (i.e. Twitter.com/BenParr) not only makes it easy to share with others, but helps you claim a little piece of social media space as your own.
Facebook has now increased the minimum number of fans required from 25 to 100. No reason was given for the change, which occurred in the Facebook Help text today.
Update: As a number of commenters note, one requirement on vanity URLs, specifically for Fan Pages, is that they must have at least 1,000 fans and have been created before 5/31/09. User profiles created today after 3PM EDT will also not be immediately eligible. Facebook explains here.