The URLs might go through a bidding process, so that whoever bids the highest gets access to a particular shortcode. The site currently only offers them to selected high-profile users to encourage them to use the site more.
Most sites usually take a ‘first-come, first-served’ approach to usernames, but that’s resulted in many companies having trouble getting their trademarks because they’re being squatted on by other users. Companies that try and shift the squatters often gain intense criticism.
Would you pay for a vanity URL on Facebook? If it wasn’t too expensive, I think I might. Let us know your thoughts on Twitter – message @techdigest.