Within two years Facebook has added more than 600 million users. While that averages around 600,000 sign-ups per day, new data suggests that the social network’s growth rate is slowing down.

According to data from Optim.al – the Facebook ad-testing and ad-management system – Facebook’s growth rate is declining. From July 2010 through March 2011, the social network averaged 8.39 user sign-ups per second. It dropped to 7.02 from March to September 2011, and fell again to five during September through December 2011.

The data also found that the U.S. accounts for 19.7 percent of the total user base. The social network added more than 97 million users in the U.S. from April 1, 2009 through December 28, 2011; however, even that has slowed with only four percent of new users coming from the U.S. in the past 90 days.

Despite these numbers, it’s business as usual at Facebook. The drop in sign-ups can be attributed to saturation. AllFacebook points out that most Internet users that are likely to be Facebook users already have accounts, restricting sign-ups to new Internet users. Focus shouldn’t be placed on the drop, but rather Facebook’s international growth.

Brazil has seen a lot of growth and added 34.8 million users between 2009 and 2011. Over the past 90 days 3,343 users were added per hour. The country – which accounts for 4.4 percent of Facebook’s user base – now accounts for 19 percent of new users over the last three months. Additionally India – which accounts for 3.4 percent of Facebook’s user base – is seeing growth with nearly five million new accounts created over the past 90 days.

While the growth isn’t staggering, it’s still great for companies with an international presence. As Facebook continues to establish itself abroad, it offers you an opportunity to increase awareness through Facebook Pages and Apps, ultimately driving more traffic to your website. U.S. businesses have seen results from long-term social media strategies and therefore could have an advantage in international markets.

[Image credit: Joelk75]