We’ve been closely tracking shifts in Facebook’s US demographic growth for a long time now, but we’re seeing an interesting trend for the first time this month. According to the latest Facebook stats, many more males 18-25 and 26-34 are joining the site than females in those age ranges, while many more women 45-54 and 55-65 are joining Facebook than men that age. Growth numbers are even in the 35-44 age range.

As you can see, nearly 850,000 more American men 18-25 were active on Facebook in August compared to July, while the number of women 18-25 grew by just under 570,000. But in the 45-54 category, the trends were reversed: the number of women active on Facebook during August grew by over 625,000, compared to growth of around 400,000 for men.

Despite the trends in raw growth, in percentage terms, growth in Facebook’s male audience actually outpaced growth in Facebook’s female audience in neearly every age category. Surprisingly, despite the fact that women continue to outnumber men on Facebook by a 57% to 43% margin, men are closing the gap.

Women still outnumber men in every age category on Facebook – most dramatically in users over 26. For example, there are now over 10.4 million US women 26-34 active on Facebook each month, compared to only 7.9 million men. And nearly twice as many women over 55 are on Facebook: 3.28 million to 1.9 million men.

What do the shifts mean for developers and marketers?

While Facebook has experienced massive growth in its US audience over 35 this year, the latest numbers show young men are migrating to the site – likely largely due to a combination of the myriad of social games on the Facebook Platform that are helping drive Facebook’s growth, as well as students make lots of new connections during the back to school season.

Despite the August trends, however, Facebook’s growth is still strong across the board, and growth rates are about the same or higher this month than they were two months ago. The following chart shows Facebook’s audience growth in the last 60 days compared to the previous 60 days. As you can see, Facebook’s growth has skyrocketed in the younger age brackets in July and August from dips earlier this summer.