To mark 8 years in existance, Twitter launched the facility to look back in time and trace your first tweet. It has posted online some amusing examples from famous celebrities.
The Atlantic has a useful article mapping trends in time. It is based on an academic paper: The Tweets They are a-Changin’: Evolution of Twitter Users and Behavior by Yabing Liuy, Chloe Kliman-Silverz and Alan Mislove
To read more academic on Twitter try the Pew and American Life project website which has examined usage of online communities.
Twopcharts has some up-to-date data which track Twitter users by language and by city.