United States
It comes as no surprise that the United States has the highest number of Twitter users by country. There are over 76 million Twitter users in the United States. As Twitter is an American company and was initially launched in the country, this should come as little surprise. The United States has always had the highest number of Twitter users, and this doesn't seem like it will change any time soon. The country has far more Twitter users than second-place Japan.
Japan
Japan has the second-highest number of Twitter users. Around 60 million Japanese people are registered on the site. As the population of Japan is estimated at around 120 million, this means that nearly 50% of Japanese people have a Twitter account. While Japan is only second in total Twitter users, they do have the highest percentage of citizens who use the site.
India
It should come as no surprise that India ranks near the top of this list, as it is one of the most populous countries in the world. It has a total of 1.4 billion residents, which makes it the most populous nation that allows Twitter. However, only 23.6 million Indians actually have a Twitter account. As more Indians gain access to the Internet, this number is sure to increase rapidly.
Brazil
Brazil is a large country with a high rate of Internet access. This results in a high number of Twitter users in the country, currently estimated at roughly 19 million. While Brazil has a high rate of Internet access, many poor people in the country still do not have an Internet connection. This means that the number of Twitter users in the country could still increase dramatically.
China
Officially, China has no Twitter users. This is because Twitter is banned in China, as are many social media sites. However, many Chinese people use VPNs to get around the ban and register a Twitter account. Even some state-run organizations, such as Chinese Central TV, have a Twitter account. The official ban on Twitter means that it is nearly impossible to come up with an accurate estimate of the number of Chinese Twitter users.
Due to the government ban and the popularity of a similar unbanned site (Weibo) it seems unlikely that Twitter will ever be extremely popular in China. Even if the government removes the ban, many Chinese people will simply continue using Weibo. Weibo offers almost exactly the same service.