Though Google’s Android operating system continues to extend its reach around the world, when it comes to app downloads, the ecosystem’s biggest market is still its home turf: the U.S.

According to a new study from market research firm research2guidance and AndroidPIT, there have been some 3.49 billion total app downloads from the Android Market since Google introduced the operating system, and half of those downloads have originated in the U.S.

But as TechCrunch reports, research2guidance’s data also reveals some other interesting tidbits. While the U.S. remains the No. 1 market for apps, it’s not growing nearly as fast as some other markets around the world. That’s likely because there are a lot of choices for apps in the U.S., with the Android Market from Google competing with the likes of GetJar and Amazon’s Appstore. And that’s not to mention Apple’s iOS operating system muscling in on Android’s market share and smaller competitors such as the BlackBerry from Research In Motion and Microsoft’s Window’s Phone 7.

Meanwhile, the second-largest Android app market isn’t a Western European nation such as the UK as one might expect, it’s South Korea. The Asian nation accounts for the most total app downloads this side of the U.S. with 603 million in total (about 9 percent of all the apps downloaded).

You might blame South Korea’s relatively high number of Android users on Samsung, a leading Android device maker that calls South Korea home. And the country is growing as an Android market, too: research2guidance suggests that a big part of that growth might be South Korea’s fast, widespread and relatively cheap cellular coverage that makes it easy and convenient to own a smartphone.

In terms of downloads each month, South Korea takes third with about 4.5 percent of the Android apps downloaded. In front in the monthly category is the UK (which takes third in total downloads with around 680 million), clocking in at just under 7 percent of monthly downloads. In second is Germany with about 6.7 percent of monthly downloads; it takes third in total downloads with just approximately 380 million apps downloaded.

The study also finds that while the U.S., South Korea, the UK and Germany might account for the largest app markets by volume, they’re not necessarily the app markets where users download the most apps. In fact, that honor belongs to Sweden. While the country doesn’t have as a high an Android smartphone penetration rate as other countries, the firm reports that smartphone users there tend to download more apps per smartphone user, around five per month. By comparison, Germany notches about 4.5 apps downloaded per person per month; South Korea and the UK each account for roughly 3.5 apps per person; and the U.S. gets fewer than 2.5 apps per person.

According to research2motion, this means that there are several different kinds of Android app markets out there, with some capable of becoming “cash cow” markets in which there is a large number of Android users and those users are downloading a lot of apps. The U.S. and some European countries, on the other hand, might be considered “saturated markets,” where there are a lot of smartphones but not a lot of apps being downloaded, and therefore relatively less money to made.

With research2guidance’s data, app developers may be able to find better targets for their apps by looking to other markets. Android developers often complain about low sales because getting noticed in the Android Market can often be difficult. Perhaps focusing on a different segment of the Android population will help more developers become more successful in developing Android apps, which will encourage more development and more quality Android software.