Navigational search definitely has a lot to do with the graph. I don't think that's computer illiteracy, though. That's just how the normals use the internet ;) We techies are the fringe.
Side note: As much as a I cringe at using google instead of the URL bar when you know the site you want to go to, I find myself doing it far more often on my phone. For example, on a mobile device, it's easier to google for "Hacker News" than type "news.ycombinator.com"
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.