Kantar Media, a WPP subsidiary, has released esimates that suggest a handful of big companies collectively spent well over $1bn buying Google AdWords ads up to September of this year.

Leading the pack is IAC/InterActive Corp., which has spent just short of $175m this year according to the firm’s estimates, while Amazon and AT&T both spent upwards of nine figures.

For many businesses, Google AdWords has become an indispensable digital marketing tool. It’s no surprise why; ads can be targeted granularly, you pay only for performance, it’s easy to track conversions and Google searches deliver intent like few other mediums can.

All of these things have made Google AdWords a hit with small and medium sized businesses, most of which can’t afford to buy expensive ads on television or in print. But that doesn’t mean that big business doesn’t love AdWords too.

To make the list of the top twenty biggest AdWords spenders in the US, a company would need to spend more than $40m a year on ads with Google. So it’s not surprising that the list is filled with tech and internet giants like Expedia (#4), Microsoft (#6), eBay (#7) and Priceline.com (#8), as well as large financial services firms such as Experian (#5), Capital One (#9) and State Farm (#10).

While it’s no surprise that companies like these are active buyers of AdWords ads, the estimated amounts they’re spending are impressive nonetheless and this serves as a powerful reminder that AdWords is just as important to major brands as it is to smaller companies that may have few other advertising tools online.

Even though these are just estimates, this also shows that Google still has a huge lead over potential rivals, including the one that attracts the most attention – Facebook. The world’s largest social network, of course, is increasingly popular with major brands because of the massive audience it delivers.

But Kantar Media’s AdWords estimates put things into perspective, Google’s twenty most prolific AdWords buyers in the United States have spent in nine months on a single product a quarter of what Facebook is predicted to generate in global revenue for all of its products in 2011.

As Facebook inches closer to an IPO that could see it valued at nearly half of Google’s market cap, that’s worth keeping in mind.