Google is the first Internet company to reach 1 billion unique visitors in a single month, according to data from comScore. Since this number comes from Google owned sites such as the Google search engine, Google email and YouTube, this doesn’t come as much of a surprise.

This milestone was hit in May and over the past year, Google’s unique visitors have increased 8.4%. Microsoft follows with 905 million unique visitors, while Facebook went up 30% to 714 surpassing Yahoo.

Here’s how The Wall Street Journal defines the process of comScore:

ComScore’s estimates are based on its “global measurement panel” of two million Internet users, similar to how Nielsen measures television ratings. ComScore refines the estimates with “page view” data that it receives from more than 90 of the 100 publishers of Web content, but not from Google.

Although Google has the most numbers, marketers are starting to look at a visitor’s time on a site as being extremely important when it comes to how they spend their ad money. Who has the advantage there? Obviously Facebook.

Speaking of Facebook, when do you think they will reach 1 billion unique visitors a month?