In it's first quarter 2011 earnings call, Google let it be known that over three billion apps have been downloaded from the Android Market since it opened. The tally reflects a 50% increase in Android app downloads since the last quarter of 2010. Google didn't go into details over what apps had been downloaded multiple times, meaning that the tally could be reflecting a percentage of apps that have been re-downloaded to replace and app that had been deleted. But even with that possibility, it's still an impressive tall.
Google also reported that they are currently activating more than 350,000 Android handsets every day. Doing the math, that's nearly 128 million smartphones a year. With that kind of traffic, it isn't surprising that Android's 3 Billion app download mark happened so quickly. And with competitors like the Amazon App Store for Android and proprietary app stores from Samsung, Sony and other handset providers, it could be far higher than Google's estimates. It also means that the 4 Billion mark will likely happen sooner than 3 Billion did.
According to AppBrain, there are currently nearly 170,000 apps in the Android Market alone. Of those, the App site rates about 38% as being of low quality apps. Google has been known to remove under performing or useless apps every quarter, so the number tends to fluctuate. But the trend is ever moving upward with quality continuing to gain ground. That's really going to be the key to continuing Android's upward trend in downloads.