Facebook has been showing signs of a decline in the US, and the latest statistics confirm the popular social networking site has been surpassed in Web traffic – for the second time this year.

If you’ve done a search on the Web lately, you may have stumbled upon the so-called discovery engine StumbleUpon, which has exceeded Facebook in US Web traffic for the month of June. The popular social networking site has at times accounted for more than half of the traffic sent out by the top seven social media websites, according to the latest figures released by StatCounter.

StumbleUpon accounts for 49.7 percent of all traffic in the US, with Facebook holding second place at 37.1 percent, according to StatCounter’s July 2011 figures. YouTube, Reddit, Twitter, Fark and LinkedIn rank among the top seven social media sites in the US tracked by StatCounter.

This isn’t the first time StumbleUpon surpassed Facebook in Web traffic; in January 2011, the intuitive search engine accounted for 43.3 percent of all US Web traffic.

While the United States still ranks as its fastest growth audience, Facebook traffic in the US declined significantly last month, according to the figures released by Inside Facebook, a site dedicated to tracking the social networking site for developers and marketers.

As its user ranks approach 700 million worldwide, Facebook traffic in the US dropped off by nearly 6 million users, according to statistics from Inside Facebook Gold analytics service, the research arm of Inside Facebook, which gets its information from Facebook’s advertising tool, TechZone reported last month. The number of US Facebook users fell from 155.2 million to 149.4 million during the month of May alone.

All of this math boggles the mind, considering Facebook is approaching 700 million users, meanwhile StumbleUpon maintains that more than 15 million people use the service to find relevant Web content.However, 500,000 new users are signing up for StumbleUpon each month.

If history is any indication, Facebook is likely to regain the top spot, but StumbleUpon could continue to be a reckoning force; in March, the San Francisco-based company announced it completed a $17 million Series B round of financing. Company officials said the capital would be used “to continue to invest in the company’s core discovery technology, expand to new platforms and grow the employee base.”

Source: StatCounter Global Stats - Social Media Market Share