Facebook may continue to gain users, but the world's biggest social network isn't gaining them as quickly as it has been, and is actually losing users in the US.

Facebook gained 11.8 million more users last month alone, according to a study by Inside Facebook. While that's a lot of new users, it's less than the 13.9 million new users who joined the site in April, or the 20 million gain during some months in the past year.

And while Inside Facebook reports that the social network is approaching 700 million users worldwide, the number of US users has dropped. The study found that Facebook lost 6 million US users in May. The study noted that Facebook had 155.2 million US users at the beginning of May but 149.4 million at the end of the month.

Other countries showed losses as well. Canada dropped by 1.52 million and the UK dropped by more than 100,000, but the US showed the biggest loss for Facebook.

A Facebook spokesperson said the company is pleased with its growth.

"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. 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 on any given day."

Dan Olds, an analyst with The Gabriel Consulting Group, said he's not surprised that Facebook is having a dip in growth and he doubts that Facebook executives are worried about it.

"Currently, Facebook has more than 150 million users in the US, which is more than half of the total population. That's a very big number," said Olds. "The growth rate is naturally going to tail off when penetration gets that high. We're also going to see some ebbing and surging over time as the user base stabilises and matures."

He added that it's probably a simple matter of people wanting to try out the site and then losing interest in it after a while. It's more of a sliding away than a mass exodus.

"I don't think this is necessarily troubling for Facebook at this time," said Olds. "Some might see this as a sign of doom, but I don't... If there were a strong Facebook competitor that was growing at the same time as Facebook was shrinking a bit in the US, it would be a different story. But that isn't the case."