The graphic above comes from the Information is Beautiful site, which incidentally is a blog in which things are done visually. Basically statistical information, of the more intriguing kind, is turned into graphs - or graphs are sourced from elsewhere. Well worth a look.
I first came across the Twitter graph via the Flickr.com site, though I don't think that I came across it directly there. Wherever it was I found it, I took a note of the URL, and that was that.
The graph is interesting, and confirms what's generally thought about Twitter. 20% of the accounts are dead - people tweeted once or twice and that was that. An enormous 50% - those in the green study - only tweet occasionally.
Only 5% have more than a 100 followers - I currently have 43, most of whom I don't actually follow myself - and there are 5% loud mouths, people who do virtually all the talking on Twitter. The other 20% isn't specified, so I can only assume it's the sort of category I come into: fairly busy (two or more tweets a day) but not exactly hugely visible.
By the way, if you want to see how little people understand graphs or maths, check out some of the comments in the Flickr.com link; it's pretty scary.