I plan to write a longer post on my thoughts on this issue, but I am pushed for time right now, so this will be brief.
Facebook's decision to allow vanity urls/usernames is creating a landrush and is against its most basic premise: all members are there with their real identity. This can lead to spam-like messaging proliferation. There have been discussions on how this will be good for SEO but that supports my point. SEO is gaming the system. In a liquid search environment, there should not be need for SEO.
The same's true for Twitter. The landgrab of twitter usernames erodes Twitters authenticity. And social media needs to be authentic to succeed.