Google's Gmail has surpassed AOL as the third most visited e-mail service in the U.S. and is poised to pass Windows Live Hotmail in about seven months.

Between July 2008 and July 2009, Gmail's number of unique monthly visitors in the U.S. increased from 25.3 million to 36.9 million, according to ComScore.

Gmail's rate of traffic growth has been increasing, too. In the July 2008 to July 2009 period, Gmail grew at a rate of 46%, up from 39% during the period between September 2007 and September 2008.

From July 2008 to July 2009, AOL's monthly visitor total declined by 19%, from 45.1 million to 36.4 million. Windows Live Hotmail, which lost 4% of its visitors between September 2007 and September 2008, managed to eke out a 3% gain during the July 2008 to July 2009 period.

But with 47.1 million monthly visitors, Windows Live Hotmail is more or less where it was in September 2007, when its monthly visitor share stood at 46.2 million.

If current trends continue, Gmail should surpass Hotmail by the end of February next year and take second place in visitor traffic behind Yahoo Mail. The release of Windows 7, however, may contribute to renewed interest in Microsoft services like Hotmail and may delay Gmail's move to second place.

Yahoo Mail, the leading free e-mail service, has been doing better lately. Its visitor traffic, 106.1 million last month by ComScore's count, grew at a rate of about 11% in 2008 and at a rate of 22% between July 2008 and July 2009.

There are of course other metrics by which one can measure the popularity of e-mail services, like the number of registered accounts. Online traffic however can be correlated with active usage.

Gmail's torrid growth coincides with a period of aggressive innovation. Google has delivered new Gmail features and capabilities every week, more or less, since the opening of Gmail Labs in June last year.

Google has also been encouraging businesses to start using Google Apps, which includes Gmail as well as online applications like Google Docs, Google Calendar, Google Sites, and Google Video.

Google did not immediately respond to a request to confirm ComScore's figures.

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