A map showing the most popular social networking sites by country according to Alexa and Google Trends for Websites traffic data
Facebook with 1 billion active users has established its leadership position in 127 out of 137 countries analyzed. One of the drivers of its growth is Asia that with 278 million users, surpassed Europe, 251 million, as the largest continent on Facebook. North America has 243 million users, South America 142 million. Africa, almost 52 million, and Oceania just 15 million (source: Facebook Ads Platform). In the latest months Zuckerberg’s Army conquered Armenia, Kyrgyzstan, Latvia and Vietnam (update: my data are about visitors, but I’d like to report that Draugiem’s spokeperson told me that they have 1.2 million members in Latvia. Facebook Ads claims 400K members)
In Russian territories there was a long battle between two main local players: V Kontakte and Odnoklassniki. According to Alexa the first is gaining ground and now has 190 million registered users.
In China QZone still dominates the Asian landscape with 552 million users, followed by Tencent Weibo and Sina Weibo. Since the latest update QZone has also conquered South Korea.
In Iran, where it’s hard to access Facebook due to state censorship, the leader is Cloob.
Source: Vincos.it
The Main Street Analyst 
It shows most of all South Korea use Qzone, but this result is totally wrong. As Korean who use social networking actively, it’s difficult to understand this result. Qzone was even made in Chinese.
Greetings to South Korea. Thank you for your comment. I took a look at this report again and it seems the indication is more that Chinese Qzone is just making its way into Korea, but seems to grow quickly. Which site do you see as the major leader in South Korea?
Korean usually use Facebook and Twitter.