Facebook launches their own URL shortener, much like http://bit.ly, http://tinyurl.com and others, except for the fact that you can’t really use it yet, unless you use Facebook on a mobile device. Those devices are automatically using the shortener; I’m assuming it will be deployed to a wider audience, with tools, etc. at some point.
You can, however, hit http://fb.me/facebookusername and that will resolve ok. I have no idea if that URL will work – it just may belong to someone who grabbed that vanity URL. I do know that http://fb.me/brianhoney works just fine.
(I just checked, the facebookusername ID was not taken.)