Apple's App Store offerings this week exceeded the 20,000 mark, as the number of daily releases seems to follow an exponential curve, according to tracking statistics charted by Apptism. The company reached 15,000 titles less than a month ago, up from the announcement of 10,000 apps on December 5th. In comparison, the App Store only contained a few hundred apps when it was opened for business seven months ago.

Customer downloads appear to have increased proportionately with the available apps, reaching 500 million downloads last month. The store nearly reached 300 million downloads by October and added another 100 million in the next six weeks.

Barclays analyst Ben Reitzes has suggested that the App Store adds to the "stickiness" of the iPhone and iPod touch, referring to the lock-in effect that is likely to prevent owners of the mobile devices from ever jumping to a competitor's products. "We believe 'apps' personalize iPhones to levels that competitors cannot match," he said. [via Macworld]