There's an additional layer of mapping added when using shortnames - the user has to remember a whole list of "@donkey2001,Joe Smith" tuples; most people (and especially non-geeks) like and operate better with fewer layers of abstraction.

The usage of a full name on Facebook makes explicit the roughly one-to-one correspondence between accounts and actual-people that is expected by users and required by the service; Twitter users openly create multiple profiles with different names and even identities, and the service and tools encourage that. In that sense, Twitter has more of a sense of a masquerade or Halloween party and less like a business meeting or high-school reunion.

Each has its strengths and its place, and the longer I use both, the less I feel like there's direct competition between the two.

A study (http://www.pewinternet.org/Reports/2009/17-Twitter-and-Statu...) did find that 26% of blacks vs. 19% of whites use "Twitter or another status update service", but it also found a large correlation with age. Since black demographics skew younger than white demographics, my guess is that the 7% racial gap would disappear if you controlled for age.