Figures from social media blog Inside Facebook revealed that Facebook lost more than six million users in the United States, falling from 155.2 million at the start of May to 149.4 million at the end of it. During the same period, the number of Canadian users also fell by 1.52 million down to 16.6 million. Without a doubt, these are potentially damaging findings as the company seeks to go public in early 2012.

But Zuckerberg and Co., who usually don’t bother to comment on third party statistics, shot back at these reports.

“Some of these reports use data extracted from our advertising tool, which isn’t designed to be a source for tracking the overall growth of Facebook,” the company said in an official statement. “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 in any given day,” it added.

Facebook’s claims are backed up by data from web measurement firm comScore, which reveals that active Facebook users are still growing in every market.

According to comScore, the #1 social networking site in the world had seen a 21% growth for American users on Facebook during May, and the average amount of time spent on the site was also up from 21 minutes per day to 25 minutes.