In our initial Twitter report published last June, one of the areas that explored were the countries and cities where Twitter was being used. Not surprisingly, nearly two-thirds of unique users (62.1%) were located in the U.S., while the U.K. and Canada were a distant second and third.

Given how much Twitter has grown over the past six months, we wanted to take a new look at global usage to see if there were any major changes. To gather this insight, we explored our Twitter repository between mid-October and mid-December, 2009 in terms of location (country/city) and overall activity (how frequently people tweet).

For location, the most significant difference in users was a surge in Brazil, which accounted for 8.8% of unique users, an increase of more than four-fold from 2% in June. Twitter users in Indonesia accounted for 2.4% of the population, compared with less than 0.5%, while Twitter users in Germany rose to 2.5% from 1.5%.

Meanwhile, the number of U.S. unique users was 50.8%, a sharp drop from 62.1% in June. This suggests the use of Twitter outside the U.S. has experienced significant growth over the past six months.

Here’s a table showing unique Twitter users around the world from Oct. 16, 2009 to Dec. 16, 2009. We explored 13 million unique Twitter accounts that demonstrated tweeting activity within that period.

(Note: In doing our research, we discovered that almost no one is using the geo-location API tool that Twitter launched last August, with only 0.23% of all Tweets tagged with geo-location. To determine someone’s location, Sysomos uses proprietary technology to infer geography based on user disclosed information which does not rely on Twitter’s geo-location API.)

And here’s the chart from our June, 2009 report.

In addition to looking at the number of unique Twitter users around the world, we also explored the geographical break-down based on the total number of tweets to determine if there were any differences between the two.

The biggest changes are in the U.S., which accounted for 50.9% of unique Twitter users and 56.6% of total tweets, suggesting that Twitter users in the U.S. are more active than many users around the world.

U.K. users accounted for 8.1% of total tweets, compared with 7.2% of unique users, while Brazilian users account for 6.73% of total tweets compared with 8.79% of unique users.

As well, we looked at the leading cities in which Twitter is being used. London has the biggest population in terms of unique users with 2.08%, while Los Angeles has 1.63%, Sao Paulo (1.47%), New York (1.44%) and Chicago (1.2%).

The table below shows the leading cities by the number of tweets. New York ranks first with 2.37% of all tweets. This compares with the table above in which New York ranks fourth in the number of unique Twitter users. This shows that New Yorkers are very active Twitter users.

Update: For a more extensive list of the countries and cities with the most Twitter users, check out this blog post. You can find all of our social media reports and whitepapers here.