Update, 6/14/11: Be sure to read our follow-up post on recent Facebook traffic according to third party measurement firms.

As we note below, we’ll need to wait to see what the long-term trends really are before knowing if Facebook is continuing to grow in the US and other countries.

Update, 6/15/11: More data is out from third party firms about May traffic. Our analysis here.

Facebook is still growing towards 700 million users, having reached 687 million monthly actives by the start of June, according to our Inside Facebook Gold data service.

Most of the new users continue to come from countries that are relatively late in adopting Facebook, as has been the trend for the past year.

But overall growth has been lower than normal for the second month straight, which is unusual.

The company gained 11.8 million more people over May, following 13.9 million over April. In contrast, it grew by at least 20 million new users over the typical month in the past 12; while there have been a few months that have registered lower growth numbers, they have not been back to back.

Why the drop? Most prominently, the United States lost nearly 6 million users, falling from 155.2 million at the start of May to 149.4 million at the end of it. This is the first time the country has lost users in the past year. Canada also fell significantly, by 1.52 million down to 16.6 million, although it has been fluctuating around that number for the past year. Meanwhile, the United Kingdom, Norway and Russia all posted losses of more than 100,000. If these countries — most of whom had adopted Facebook many years ago — had not lost users, and instead posted even small gains, Facebook would have had a much more typical month.

Going forward, we’ll be watching closely to see what longer-term trends emerge. 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.

Still, by the time Facebook reaches around 50% of the total population in a given country (plus or minus, depending on internet access rates in that country), growth generally slows to a halt, as we’ve noted before. So far, Facebook has been able to make up stalls and losses with big gains in heavily-populated developing countries like Mexico, Brazil, India and Indonesia. As you can see in the table above of the ten countries that gained the most users over May, this continues to be the case.

But how much further can it go if it is to reach its goal of 1 billion monthly active users? At least without getting into China — a move that as we and many others have noted, could both give it access to hundreds of millions of users and compromise its reputation in the US and many other countries around the world.