From The Next Web:
A recent study by Semiocast has revealed that while the US leads with the most number of Twitter accounts, currently standing at 107.7 million, the Netherlands has the most active users.
Analyzing 383 million Twitter user profiles that were created before January 1st, 2012, Semiocast determined that US Twitter users account for 28.1% of all users, while Brazil snagged the second spot with 33.3 million accounts, ahead of Japan with 29.9 million.