Facebook is said to have suffered a second successive slump in users in America and Britain, leading some to suggest people are tiring of the ubiquitous social media site. Worldwide, user numbers are nearing 700 million, according to Inside Facebook , which tracks its popularity - but it also says Brits and North Americans are turning off in droves. Facebook aims to attract one billion users - it currently has around 687 million - but has reportedly lost 100,000 British users overall in May. Commentators think this is because Britons - and six million in America, who also closed their accounts last month - are suffering from 'Facebook fatigue'. US user numbers slumped to 149.4 million in May, down from 155.2 million. In Canada there was a drop of about 1.5 million, and numbers also decreased in Russia and Norway. It is the second month in a row that Facebook growth has stalled - the figures suggest that there could be a natural limit for the social networking site. In the first half of the year executives announced that there were 30 million accounts in the UK - about half the population. That number was an increase of about four million from last July and was the highest proportionally in Europe. However, the fact that it now has 100,000 fewer British users than a month ago should be a concern for founder and CEO Mark Zuckerberg . There is speculation gathering that this could be due in part because the social networking site, which launched in 2004, has not recently added anything new to keep users entertained. In addition, many have concerns about privacy. Last week, Facebook attempted to roll out a facial recognition system which caused an outcry. However, it is picking up new users in developing countries like Brazil and Mexico, for example. A bullish statement from the site said: "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 in any given day."