• by Gavin O'Malley  @mp_gavin, April 22, 2009

Further evidence has been found to support the increase in social networking activity amid the flat growth of email usage.

Comparing usage on Windows PCs of the top web mail sites and MS Outlook to usage of leading social networking site Facebook.com, technology usage metrics provider ClickStream has found that usage of Facebook nearly doubled among adult U.S. Internet users while email usage remained flat.

From October through the end of February, usage of Facebook.com climbed from 16% to 32% among U.S. adult Windows Internet users, while usage of MS Outlook remained flat.

Among users ages 18-24, the disparity between MS Outlook and Facebook.com usage on Windows PCs was more striking. While MS Outlook usage averaged about 14% penetration per month for the same period, Facebook.com usage averaged 38%.

Considering that MS Outlook is often used more by business users than consumers, the study also compared all U.S. Windows users' use of web mail sites such as Google Mail and Microsoft Hotmail.

Similar findings showed a steady combined penetration of about 80% during the study, but a decline in clicks and average minutes that users spent interacting with the sites in these months.

The Facebook.com penetration rates were even more prominent among U.S. Macintosh computer users, for whom Facebook.com usage penetration climbed from 51% to 63% for the same period, to nearly twice that of Windows PCs.

Similar to Facebook.com, the micro-blogging site Twitter.com is experiencing rapid growth among US Windows PC users, with significant growth in the first quarter of 2009.