It took Google 20 months to reach the 1 billion apps installed on the Android platform. Five months later, the company had hit the 2 billion apps installed milestone. Now it's at 3 billion apps installed. At this rate, we'll probably see it hit the 4 billion and 5 billion marks next month.

During Google's first-quarter earnings call today, senior vice president for commerce and local Jeff Huber said the platform had grown from 2 billion apps installed to 3 billion apps installed in just two months. Huber also noted that app installs are up by 50 percent quarter-over-quarter (from Q4 2010 to Q1 2011) and that Android was seeing 350,000 device activations per day.

To put the 3 billion number into perspective, Apple saw 1 billion apps downloaded in April 2009. In April 2011, the company passed the 10 billion apps downloaded mark. That's three times more than what Google has now, but the growth is not at an increasing rate.

In short, because Android phone sales are growing so quickly, Google's app downloads are going to soon catch Apple's numbers. That's impressive, given that Google's Android Market has over 200,000 apps as of December 2010 while Apple's App Store has over 300,000 apps as of September 2010. It's also expected that the number of apps for Android will also pass the number of apps available for iOS.

Because Android is open, however, the platform's users can download apps from more than one app store. For example, Amazon launched its own Android App Store just last month.