Facebook Insider reported that 6 million users left the social network in the US, 1.5 million left in Canada, and 100,000 left in the UK for the month of May 2011.

These figures suggest that Facebook’s explosive growth may be hitting a wall in developed countries.

A Facebook spokesperson promptly downplayed Facebook Insider’s data and said the following:

From time to time, we see stories about Facebook losing users in some regions. Some of these reports use data extracted from our advertising tool, which provides broad estimates on the reach of Facebook ads and isn't designed to be a source for tracking the overall growth of Facebook.”

But regardless of what really happened in May, Facebook is hitting a demographics wall in the US. That is, Facebook is already close to saturating the US population of potential users.

As of May 2011, Facebook has 150 million US users (according to Insider Facebook), which is almost half of the US population. Moreover, it’s mostly from the population that’s likely to use Facebook, namely those under the age of 45.

A 2008 calculation from Nick Burcher revealed that 90 percent of Facebook users are under the age of 34 and 97 percent are under the age of 45.

How much of the US population is under the age of 45? Sixty-one percent, or about 190 million. Facebook, from this perspective, doesn’t have that much room to grow.

Below is an Excel file containing the data on the age of Facebook users compared with the age of the US population.

Facebook US Demographics Wall - IBTimes