Even though more than one in ten people in the world have a Facebook account, its membership in the U.S. is dropping according to a report.

According to Inside Facebook, which tracks the social network's membership, Facebook's membership rate in the U.S. dropped four percent in the month of May. This drop equated to nearly six million users. Canadian membership also dropped significantly in May, with 1.52 million users signing off for good.

Other countries that posted membership significant losses were United Kingdom, Norway and Russia. Top gainers were in developing countries with large populations such as Indonesia, Brazil, Mexico and India. Overall, its membership did grow 11.9 million members in May, but this is down from the 13.9 million users who joined in April.

Inside Facebook couldn't really give an exact reason as to why this may have happened. It did offer a few suggestions and said it could simply be a short term thing.

Bugs in the Facebook advertising tool that we draw this information from, seasonal changes like college graduations, and other short-term factors, can influence numbers month to month and obscure what's really happening, Eric Eldon, from Inside Facebook, said in a blog post.

In addition, the research firm says by the time Facebook reaches half of the population in a particular country, membership actually grinds to a halt.

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. We are very pleased with our growth and with the way people are engaged with Facebook. More than 50% of our active users log on to Facebook on any given day, a Facebook spokesperson said in an email.

It will be interesting to see Facebook's strategy once it taps out membership in the aforementioned countries. The country has a goal to reach one billion users, and it also wants to show to possible investors for an IPO, that it hasn't tapped out for possible members.

One possibility is an expansion to China, which with its one billion people, would certainly be a fertile ground for the company's growth aspirations.

Follow Gabriel Perna on Twitter at @GabrielSPerna