About 6 million U.S residents stopped posting on Facebook in May and thus in the country where the social networking site was created the number of users dropped from 155.2 million to 149.4 million.
On the global level, the social network continues to attract users albeit at a slower rate than in 2010, and so its creator - Mark Zuckerberg - will have to wait a while longer than planned to announce that it has surpassed the anticipated figure of 700 million users.
Facebook is obtaining its best results - in terms of new users - in Latin America, where Brazil is leading the social network's growth with a jump in users from 17.1 million in April to 19 million in May.
The South American giant is followed by Mexico, which during the course of the same one-month period went from 23.7 million users to 25.6 million.
The social network's growth rate in 2010 on the global level was 20 million new users per month, on average, a figure that had fallen to "just" 11.8 million per month so far this year.
Analysts say this decrease in the growth rate is due to the loss of users in key markets such as the United States and Canada, as well as to the fact that the number of potential users in each country has begun to peak.
The appearance of national social networks also has hurt Facebook in some countries like Russia, where 100,000 users have dropped out of the mix.
In Spain, Facebook is winning the marketing battle with homegrown social network Tuenti, despite the latter's high popularity among teenagers.
Tuenti has 11 million members, while Facebook grew last month by 32.2 percent to 15 million users.