As its user ranks approach 700 million worldwide, Facebook traffic in the United States has dropped significantly, a sign that the social network behemoth may be on the way down, reports Inside Facebook.

According to statistics from Inside Facebook Gold analytics service — which gets its information from Facebook’s advertising tool — new Facebook users totaled 11.8 million people in May — an impressive number by any stretch of the imagination. But that compares to the 13.9 million who joined in April, and an average of about 20 million in the 12 months prior.

This drop is primarily due to a plunge in registered users in the US. The number of US Facebook users fell from 155.2 million to 149.4 million during the month of May alone. That’s nearly 6 million people who decided they no longer need to stay connected through the service, and is the first time Facebook has lost users in the last year.

Facebook-Traffic-May-2011

It’s not looking good in the rest of North America, either. About 1.5 million Canadians also ditched their Facebook profiles, bringing that country’s total users down to 16.6 million. This may or may not mean anything, however, as Canada’s userbase has fluctuated in this area over the past year. Other countries where Facebook had losses of 100,000 or more include: the UK, Norway and Russia.

Facebook made its biggest gains in Latin America, specifically Mexico and Brazil, as well as Asian nations, like India, Indonesia and the Philippines. And worldwide numbers show that Facebook’s traffic is still rising rapidly.

But for Facebook to continue the meteoric climb its enjoyed in recent years, Inside Facebook says the company must break into the populous Chinese market, which would give access to hundreds of millions of users who have yet to join the site. And you know what? It just might do exactly that. Though it’s far from clear how it would compete in this highly competitive — and censored — market.