Despite being one of the largest Internet companies in the world, Google keeps on growing with some products getting faster adoption than others. Gmail has been around for five years now, or two if you count from the official public release, as before 2007 Google's email service was invitation only, but it's still growing at a steady pace. According to comScore numbers, released by TechCrunch, Gmail passed AOL Email in the US in July with 37 million unique visitors becoming the number three web mail service in the country.

In the first half of 2009, Gmail's unique monthly visitors, a very good estimate for active monthly users though not 100 percent accurate, grew 25 percent – no small feat considering its size. Actually, registered user numbers are of course higher than the 37 million that used the email service in July. The record visits mean Gmail has just passed AOL Email, which drew in 36.4 million visitors last month. AOL Email, like many of the company's “legacy” services and products, has been bleeding users for years and, in the first six months of the year, unique visitors declined by 22 percent.

In fact, at the end of 2008 AOL had more visitors than the current number two email service, Microsoft's Windows Live Hotmail, which only grew 8 percent since January. Still, Hotmail enjoys a solid lead over Gmail, having reached 47 million unique visitors in the US last month. Of course, none of them really have a chance of getting close to the undisputed leader Yahoo anytime soon. Yahoo Mail had 106 million unique visitors in the US in July, more than double that of its closest follower Hotmail, and, despite its size, it still managed to grow 16 percent this year.

Of course, while they are a valuable metric, unique monthly visitors don't really say much about how frequently a service is used. One issue is that there is bound to be a significant overlap, with users having more than just one email account from one provider. But the real problem is that a user who visited Gmail 50 times and visited Yahoo Mail only twice would have the same value from the unique visitors perspective, though it would be clear that the particular user greatly prefers Google's email offering.