• by Erik Sass , Staff Writer @eriksass1, March 7, 2011

The last two years have seen spectacular growth in Facebook's U.S. user base, comparable to the huge expansion of radio and TV ownership in the early days of those media. But inevitably, like its predecessors, Facebook will eventually begin to approach saturation - and indeed it seems to be reaching this stage now, judging by the flattening growth curve.

According to data from comScore, Nielsen and the company itself, Facebook's unique visitors per month in the u.s. increased from 42 million in January 2009 to 69.4 million in June 2009, then 103 million in January 2010 - adding 61 million unique visitors for a percentage increase of 145% in one year. In terms of penetration, this means Facebook's reach jumped from 13.6% to 33.4% of the American population over this period.  

However, between January 2010 and November 2010, Facebook added "just" 49 million unique visitors in the U.S., for a percentage increase of 45.7% over this period. While this is still an impressive gain, a closer look at month-to-month data leaves no doubt that Facebook's growth is flagging in the U.S. After adding unique visitors at an average rate of 7 million per month in the first half of 2009, Facebook's rate of growth slowed to an average 5.2 million per month in the second half of 2010.  

But just as its U.S. growth is lagging, Facebook's growth abroad is kicking into high gear. Facebook's total non-U.S. user base increased from 108 million in January 2009 to 257 million in January 2010, matching the rate of increase in the U.S. over this period. By November 2010, the number of non-U.S. users soared to 496 million, a 93% increase in just ten months - twice the U.S. growth rate. Looking at the pace of new additions, from 12.4 million new non-U.S. users per month in 2009, Facebook's overseas growth rate jumped to 23.9 million new non-U.S. users per month in 2010.

Unsurprisingly, this phenomenal expansion is concentrated in developed regions, although there is increasing penetration in the developing world. The number of Facebook users in Asia jumped from about 92 million in October 2010 to 112 million in January 2011. In Indonesia alone, the number of Facebook users grew 20.6% from 26.6 million to 32.1 million in this period.

But Facebook penetration also varies widely between countries. For example, the New York Times recently noted that just two million Japanese have joined Facebook - less than 2% of that country's population. Facebook had nine million unique visitors in Brazil in October 2010 - just 4.7 % of Brazil's 191 million people - versus 36 million or 18.8 % for Orkut. In Russia Facebook attracted just 4.5 million visitors in October, or 3.2% of the population of 142 million, trailing rivals like Vkontakte.ru, with 27.8 million visitors (19.6%) and Odnoklassniki, with 16.7 million visitors (11.8%).  In July, Facebook attracted 20.9 million visitors in India, or just 1.7% of India's roughly 1.2 billion people.