The business of sharing is slightly off.

Facebook has seen “big traffic drops” in the U.S. and Canada, even while it slowly closes in on 700 million members globally, according to Inside Facebook, which monitors membership rates of the social network. The behavior of the social network’s North American and (some) European membership suggests its growth potential in these nations is now limited. And that’s potentially a big detriment to a Facebook IPO.

Facebook’s membership contracted by nearly 6 million users in the U.S. during the month of May, falling to 149.4 million (a drop of nearly 4%). Canadian membership declined by 1.52 million in an 8% fall over the same period. Inside Facebook notes that this is typical up-down behavior for Canada over the last year, but it’s the first time U.S. figures have fallen in 12 months. Small losses were also shown in the U.K.’s, Russia’s, and Norway’s figures. Furthermore, while overall membership grew by 11.8 million in May, this is a drop over the 13.9 million new users in April, and is down significantly over the 20 million-per-month typical rate over the last year. Plus, it’s also the first time two months have shown lower growth figures back to back.

A Facebook spokesperson offered this response via email:

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.

The data’s not all grim. Mexico’s membership grew 7.6% in the month to June 2011, Brazil’s soared 10%, and India’s up 6.7%–over the last 12 months these nations have grown 94.2%, 294.1%, and 160% respectively. It’s fair to say Facebook adoption in these three nations, plus a list of developing nations elsewhere around the world, is soaring.