The question you need to ask is 'what facebook has in store for us next?' Having your own vanity url has been around for couple of years now, with LinkedIn, Flickr, SlideShare, Gmail, Twitter, Qik and many more. Being a number is so 1984, and facebook realized that. In my opinion (and others) facebook are working on something bigger, that would increase the loyalty of its friends - something like an email service.
And why the heck not? According to recent data facebook has some 200,000,000 loyal users and growing, built-in chat services, strong eco-system of developers - the only thing missing is messaging. And when you pick your vanity url you basically decide your email address: [email protected].
If you have other suggestions I'd love to hear them - drop a comment below.