Facebook has to many users that a lot of people use google to get to Facebook (some just blindly clicking on the first link: http://www.readwriteweb.com/archives/facebook_wants_to_be_yo...)
To me it seems like Twitter just hasn't spread to the computer illiterate crowd, so this kind of graph is unsurprising.
http://cdixon.org/2010/01/22/techies-and-normals/
I see the navigational search as an indication of mainstream adoption.
One other researcher I've talked to has similar results, anybody else gathering data on this?
Twitter population is about 10 times smaller than of Facebook. Also, many twitter-ers simply use a desktop/mobile client and not do anything Twitter-related on the web.
If you sort the Rank by Twitter, you'll see Brazil is the only place where Twitter outranks Facebook, which I would attribute to the Orkut dominance that pre-dated Facebook in that country.
And users are not everything (and people searching google are NOTHING). Twitter has had huge impact News and News gathering. Facebook is/has revolutionized the game industry (or created the casual games industry) POV.
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.
It would be interesting to see twitter networks broken down by demographic and an analysis of what each group uses it for. Some people use it to convey social information ("I'm in the checkout line at Walmart"), others as a stunted form of IRC ("Oh no, you did not just say that!") and most of the people I follow use it as a substitute for email where the primary purpose is to send links around.
If anyone feels compelled to do the research, I'd love to read your findings.
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.