The growth of Google's Gmail service is accelerating and, if current trends continue, it could surpass Windows Live Hotmail by the end of this year.

Between December 2007 and December 2008, Gmail's number of unique monthly visitors in the United States grew 43%, from 20.8 million to 29.6 million, according to ComScore. Windows Live Hotmail lost 5% of its unique monthly visitors during this period, falling from 45.7 million to 43.5 million.

Between September 2007 and September 2008, Gmail's visitor total grew 39%, from 18.8 million to 26 million, ComScore figures indicate. Windows Live Hotmail during this period saw its visitor share decline 4%, from 46.2 million to 44.6 million.

If Google's Gmail growth rate rises to, say, 46% over 2009, it could reach approximately 43 million unique U.S. visitors by the end of the year. And if Windows Live Hotmail continues to bleed visitors at a rate of, say, 3%, it will finish the year with around 42 million unique visitors per month.

Yahoo, with 91.9 million unique monthly visitors in December 2008 and about 11% growth last year, has reason to worry, too. If Gmail continues growing as it has been, it could become the leading free e-mail provider by the end of 2011 or thereabout.

Measuring service usage by unique monthly visitors in the United States doesn't tell the whole story, but the statistical trends suggest that Google is doing something right, something that Microsoft isn't doing and that Yahoo needs to do more of.

Part of Gmail's success may be because of Google's policy of rapid-fire innovation, a practice formalized last June with the opening of Gmail Labs, a showcase and playground for Gmail engineers. Gmail Labs takes the form of a tab in the Gmail Settings menu that allows users to try out a variety of new features, some of which are useful and some of which are just fun.

On Thursday, Google said that Gmail users now have access to multiple in-box views through a new Gmail Labs experiment called Multiple Inboxes.

That followed new Gmail-related features announced Wednesday (Latitude), Tuesday (menu bar improvements and keyboard shortcuts), and Monday (task list management from iPhone or Android phones).

January had fewer Gmail announcements but one was quite substantial: the availability of offline access for Gmail. In other months, the bounty has been greater: Last October, Gmail saw the introduction of emoticons for messages, Gmail for mobile version 2.0, canned responses, contact manager improvements, advanced IMAP controls, and Mail Goggles.

While the impact of most Gmail announcements is relatively minor, Google's relentless evangelism of its product and its near-daily innovation appears to be paying off by bringing in more users. It's as if users can tell when an online service is thriving and well-tended.

Latitude, a Google Maps feature that lets users share their location through mobile devices or Gears-enabled computers with friends (Gmail contacts), should only accelerate the growth of Gmail because sharing a location with a friend serves to encourage that friend to sign up for a Google Account if he or she doesn't have one already.

As Google explains on its Web site, "If your friends aren't using Latitude, they'll receive a location request e-mail, but they must sign into Latitude with a supported Google Account before they can accept your request."