Pew Internet released another interesting report last year about Twitter usage. It only focuses on the USA, but I imagine that usage demographics and other statistics are fairly consistent across the West, including Australia. This is the first Pew report that has focused on Twitter alone, rather than lumping it in with other “similar services”.
Flowtown has turned the data from Pew into an amazing graphic, which I’m reproducing here.

The most interesting thing – which obviously wouldn’t translate across to Australia – is the large Hispanic user-base of Twitter, compared to only 5 percent of non-Hispanic white Internet users who also use Twitter.