Half of the World's Online Population Uses Facebook

Facebook remains the world’s largest social network by a wide margin. According to GlobalWebIndex, 51 percent of the world’s internet users were active on Facebook in the first quarter of 2013.

Google+, once referred to as a ghost town, has steadily built a remarkable user base and now ranks second with a penetration rate of 26 percent.
The Top 10 are completed by YouTube, Twitter and six Chinese social networking sites.