Gmail launched as an invitation-only beta in 2004 and has grown a steady following since its release to the general public in 2007. Now, Google's web-based mail service has taken AOL's spot at number three for the most popular online email service and is showing no signs of slowing down. It is poised to overtake Hotmail - which has been around for over a decade.

According to ComScore, Gmail's monthly US visitors swelled from 25.3 million in July 2008 to 36.9 million in July 2009. Other Google services have also experienced a spurt, growing at a 46% rate this year compared to 39% in 2008 - its competitors haven't been as fortunate, though. AOL's monthly visitors have declined from 45.1 million to 36.4 million in the same period.

If Gmail's growth trend continues, the service will trump Microsoft's Windows Live Hotmail in about seven months. Hotmail currently caters to about 47.1 million visitors - a number that has remained flat in recent years. The service lost 4% of its total visitors between September 2007 and September 2008, and grew by 3% between July 2008 and 2009.

Yahoo Mail is the leading free web-based mail service, and has seen positive figures of late. Y! Mail grew by 11% in 2008, and 22% between 2008 and 2009. InformationWeek noted that the release of Windows 7 may renew interest in Microsoft services, and that Google has been encouraging businesses to start using Google Apps.