• by Mark Walsh  @markfwal, August 5, 2010

After a June swoon, Facebook's U.S. growth heated up again in July with the addition of three million active users across all demographics. The social network's U.S. expansion dropped to 328,000 users in June after adding 7.8 million in May, according to data from the Inside Facebook blog.

The unusually steep fall-off in U.S. membership in June was overshadowed more recently by Facebook in July hitting the milestone of 500 million active users worldwide. About three-quarters of its members are located outside the U.S., but its own backyard remains the biggest ad market for the company.

Last month, the increase in U.S. users in the 18-25 age bracket in particular bounced back after Facebook actually lost younger users in June. Digging deeper into demographics, the Facebook-tracking blog found the broader 18 to 44 age group that contracted in June added users last month, but not as fast as teens and older users 45 to 65.

When it comes to gender, women still outnumber men on Facebook, but men in July led growth across all age segments. "These broad trends suggest several ways to read July's growth. One is that Facebook's audience is simply becoming more reflective of the overall population over time," according to Inside Facebook. "Early on, Facebook was dominated by users ages 18-34 and had more women then men."

The site also suggests that the loss of some younger users could have resulted from the controversy around Facebook revising its privacy policy this spring. Online gaming site Roiworld earlier this month released http://mashable.com/2010/06/30/teens-social-networks-study/ findings showing that nearly one in five (19%) teens who have an account no longer visit Facebook or are using it less, showing signs of "Facebook Fatigue."

But overall it appears Facebook is simply hitting saturation level in the U.S. with 42.1% penetration of the population. "Long-term, Facebook's growth in the U.S. does appear to be on a slow decline," states Inside Facebook. Even so, it notes the company still has vast global markets to conquer and that any gains in the U.S. are far outweighed by "the aggregate additions of the rest of the world."

At the same time, comScore figures show Facebook's traffic increased this year so far from 112.4 million unique visitors in January to 141.6 million in June, making it the fourth-highest-ranked Web property behind Google, Yahoo and MSN. Despite the June "blip," Inside Facebook Editor Justin Smith says the overall trend is continued growth for the social network. It just will not be as fast in the U.S. as in the past.