According to figures tracked by Faceboook monitoring site Inside Facebook, the social networking site lost 6 million users in the US and 100,000 in the UK in May.
However, the social network has questioned how the site has arrived at the figure, but other net measurement firms said they had seen the figures grow over the period.
"From time to time, we see stories about Facebook losing users in some regions. Some of these reports use data extracted from our advertising tool, which provides broad estimates on the reach of Facebook ads and isn't designed to be a source for tracking the overall growth of Facebook," Facebook said in a 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.
According to Inside Facebook's claims its figures showed that 1.5 million Canadian users left the social network in May.
However, the overall growth continued with the numbers touching a total of 687 million users worldwide as many new customers signed up from countries such as India, the Philippines and Indonesia.