The number of new Facebook users in the United States nosedived in June, just as the popular social networking site is taking root around the globe.

Last month the site actually lost more than a quarter of a million regular users in the U.S. between the ages of 18 and 44, according to the site Inside Facebook. It registered 320,800 new members, defined as first-time users and those who logged into the site for the first time in over a month. The figure is in sharp contrast to May's 7.8 million new active users, a 6.7 percent monthly growth. (Facebook's average monthly growth in the last year has been about 3 to 4 percent - June's, however, was just .3 percent.)

Meanwhile, today 70 percent of Facebook's users are outside the U.S. Thanks in part to the company's translation efforts, the number of active users in the past year has tripled in Mexico (to 11 million) and quadrupled in Germany (to 19 million), where in February the California-based social networking site displaced the homegrown StudiVZ as the country's leading social network. And in India, Facebook now is neck and neck with Orkut, an Indian social networking site that was more than twice its size last year. Just two years ago, the site only was available in English.

"They have been more innovative than any other social network, and they are going to continue to grow," Jeremiah Owyang, an analyst with the Altimeter Group, told The New York Times. "Facebook wants to be ubiquitous, and they are being successful for now." The site currently has some 500 million users, and founder Mark Zuckerberg confidently is predicting it will reach a billion.

Nearly two-thirds of all Internet users in the US have Facebook accounts, but in June, 253,840 American members ages 18 to 44 didn't log on. The largest group of deserters - more than 100,000 - was women ages 26 to 34.

A month isn't enough to detect a pattern - especially when you consider that Wednesday  a new poll revealed 34 percent of women age 18 to 34 said checking Facebook was the first thing they did in the morning, even before washing their face or brushing their teeth. (Just over a quarter admitted to sneaking a peek in the middle of the night, according to the Lightspeed Research for Oxygen Media poll.)

Analysts say an explanation for Facebook's decline could be as simple as a glitch or a seasonal dip: June is a month of travel, graduation and weddings - all of which may keep people offline. 

"Less excitingly, the negative growth could simply be a blip. But in the years we've been tracking the demographic data, we've rarely seen a dip like this, so we would tend to favor the idea of a root cause," Inside Facebook's Chris Morrison wrote Tuesday.

He added: "One possibility is that we're finally seeing the backlash from heavy media attention to Facebook privacy issues -- some of which were real, some the result of confusion and sensationalism."
It may also be that Facebook has less room to grow: It's got a 41.1 market penetration as of the end of June.

Ray Valdes, an analyst with market research firm Valdes, told the San Francisco Chronicle the company's big challenge isn't getting new members - it's keeping them.

"They don't necessarily need to grow user population to become a profitable, highly valued company," he said. "They do need to maintain their value proposition and keep users engaged even as the novelty factor wears off. It's not about getting users to sign on, but about getting them to log on."