SAN JOSE, Calif. — Even as Facebook closes in on its 600 millionth user, its growth in the United States and parts of Europe is slowing and may be approaching the saturation point, meaning its future increase in users will be ever more dependent on the developing world.

Over the past six months of 2010, about one-quarter of Facebook’s total growth in regular users came from the U.S., but during December, just 5 percent of the social network’s new users were in the U.S., according to Inside Facebook Gold, a data service by the research firm Inside Network. Facebook gained more users in India, Mexico, Brazil, Taiwan, the Philippines and Indonesia than in the United States during December.

Facebook’s U.S. audience growth has decelerated for the past four months, raising questions among analysts about whether the Palo Alto, Calif., social network is approaching its saturation point here, and highlighting Facebook’s need to get people to spend more time on the social network in order to keep its revenue growing. A similar phenomenon is occurring in countries such as Iceland, Denmark and Sweden.

Facebook now has about 146 million regular users in the United States, according to both Inside Facebook Gold and the German-Czech analytics firm Socialbakers. Data from both show that Facebook’s U.S. growth has slowed significantly in recent months, and is on the verge of crossing the point where less than one-quarter of its users are in the United States. Facebook declined to comment on the numbers, which are roughly echoed by Internet tracking company comScore.

“It’s inevitable that in highly saturated countries growth has to decelerate,” said Justin Smith, founder of Inside Facebook, who believes it’s an “open question” whether Facebook is approaching its user limit in the U.S. “You can only get a certain percentage of the population to become active users, and as you get closer to that ceiling, you are going to slow down in terms of growth.”

Facebook now reaches a striking 73 percent of U.S. Internet users, according to comScore. Facebook user estimates from comScore, Inside Facebook and Socialbakers all translate to more than 55 percent of the total U.S. population over the age of 12, the eligible age to join the social network.

“I would say this might be the U.S. reaching an inflection point,” Jan Rezab, co-founder of Socialbakers, said of the recent U.S. growth slowdown in an e-mail message, “but only time will show if the U.S. can outgrow 50 percent penetration.”

The change does not mean Facebook — pegged at a $50 billion value after Goldman Sachs and a Russian investor recently agreed to invest $500 million — faces any foreseeable slowdown in revenue growth, experts say. But the demographic fact that there are fewer Americans every day who don’t use Facebook means the social network will increasingly need to focus on getting existing users to come back more often and spend more time. With the vast majority of its revenue coming from display advertising, more time on Facebook equals more revenue for the social network.

The good news for Facebook is that Americans on average spent even more time in 2010 sharing photos, updating their status and playing social games like Zynga’s new “CityVille.” The average time spent by each U.S. Facebook visitor grew by 30 percent, to 320.6 minutes, over the past 12 months, according to comScore, with people spending about the same amount of time each visit, but making more visits per day on average.