Facebook threw cold water on Tuesday on a report that it had lost users in the United States over the past month.

The Inside Facebook website reported Monday that Facebook lost nearly six million users in the United States in May, falling from 155.2 million at the start of the month to 149.4 million at the end of it.

But a Facebook spokeswoman cast doubt on the measurement overnight and directed AFP to the latest figures from tracking firm comScore.

"From time to time, we see stories about Facebook losing users in some regions," the Facebook spokeswoman said in a statement to AFP.

"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," she said.

"We are very pleased with our growth and with the way people are engaged with Facebook," the spokeswoman said.

The comScore figures showed unique visitors to Facebook in the United States rising from 150.7 million in February to 153.0 million in March to 154.0 million in April to 157.2 million in May.

However, Facebook did suffer declines in US unique visitors in January and February, according to the comScore figures.

The number of unique US visitors to Facebook.com dropped from 153.9 million in December to 153.0 million in January to 150.7 million in February, according to comScore.

According to Inside Facebook, which closely tracks developments and trends at the Palo Alto, California-based social network, Facebook had 687 million members at the start of June.

ComScore said Facebook had 698 million unique visitors worldwide in April.

Facebook itself does not regularly release membership figures except to announce milestones such as when it crossed 500 million users in July of last year.