I was somewhat surprised to read in The Korea Times that, for the first time ever, as of August 2011, traffic to Facebook exceeded that to Korea's homegrown Cyworld. Social networking in the form of Cyworld had been introduced in Korea a full half decade before the invention of Facebook in the United States, and it became the most popular such service in Korea for many years. All of that began to change around the time the iPhone finally made it to the Korean market, near the end of 2009. The Korea Times article provides some useful background on why the shift has taken place.
The accompanying line graph, from Rankey, (click to see a larger version) shows the dramatic shift in visitors to Cyworld (blue line) versus Facebook (red line) from January of 2010 through August, 2011.