The number of US women over age 55 using Facebook grew by 175.3% since September 2008, making mature females one of the fastest growing demographic groups on the social network, according to usage statistics released by independent blog Inside Facebook.

The number of men over age 55 also grew dramatically during the same four-month period (up 137.8%), but women over 55 still outnumber men in this age group by almost two-to-one.

Key Facebook stats as of Feb. 1, 2009, from Inside Facebook:

There are 45.3 million active US Facebook users.

The number of Facebook users are growing in every age/gender demographic.

Facebook use among women is growing faster than among men in nearly every age group.

The fastest growing age group by total users is 26-34-year-olds; 45% of Facebook’s US audience is now 26 years old or older.

Facebook’s US audience has continued to grow in recent months, fueled primarily by those ages 26+, said Inside Facebook. In particular, the network has rapidly gained popularity in the US with people 45+, growing by more than 165% among both men and women in the last four months. Read more at www.marketingcharts.com