• by Erik Sass , Staff Writer @eriksass1, March 2, 2011

It comes as no surprise that Facebook appears to be reaching saturation in its country of origin, as the massive social network now counts just over half the U.S. population as members. But this slowing growth rate makes it all the more urgent that Facebook monetize its existing audience more effectively, especially if it is to support the huge valuations assigned by analysts and private investors.

There has been an unmistakable drop-off in the number of new members added by Facebook beginning last fall: from August 2010-January 2011, in the U.S. Facebook has grown from 148 million to 153 million unique visitors, according to comScore, for an average rate of increase of about one million per month. True, there have been slow periods in the fall and winter seasons in the past -- but this time around is even slower. From August 2009-January 2011, it grew from 92.2 million to 112.4 million, adding 20.2 million new visitors, or just over four million per month.

Looking at the full year, Facebook added new visitors at an average rate of about 4.8 million per month in 2009; in 2010 that slowed to 3.5 million per month, and 2010 is off to a pretty weak start with a drop of 0.9 million unique visitors in January (the second time Facebook has ever posted a decrease in month-to-month figures for unique visitors -- the first time was February 2010, when it fell 0.4 million, also according to comScore).

Put another way, from January-June 2009, Facebook added 19.8 million unique visitors; from July-December 2009, it added 24.3 million; from January-June 2010, it added 29.2 million; but from July-December 2010, it added just 8.4 million.

On the positive side, Facebook's is still growing by leaps and bounds overseas: from January- December 2010, Facebook's total non-U.S. membership grew from about 250 million to about 500 million, increasing roughly 100% (compared to a U.S. growth rate of 37% over the same period).