The stream of data in twitter is massive. Some 55 million tweets per day. The CASA Tweet-O-Meter is measuring the speed, the landscape maps are an attempt to visualize the active locations in urban areas and many more such as the twitter steam graphs are visualising the actual online activity. Newly sprung up tool such as the tweetmetrics are trying to summarize and characterise profiles, but there is overall relatively little knowledge on twitter usage and demographics. YOu can find some overall details on techcrunch or more temporally-local infos on events such as the NY fashion week on mashable.
A comprehensive overview was put together earlier this year in April by website-monitoring with a focus on overall facts.